It's not a slippery slope.
Going to break up replies (not para by para though) - you came up with an argument about 1 in 10 people being a murderer in a specific space therefore all should be imprisoned. This is a misleading analogy to make and thus, created a slippery slope.
The consequences in your analogy are not identical to the consequences in the issue at hand, i.e. whether to exclude transwomen from women-only spaces.
The flipside of that issue is - whether to preserve women-only spaces, ensure a reasonable equality of access to them, or make sure that women who really don't want to welcome men into their spaces can be obliged.
Thus, if a woman's bathroom is turned into a unisex bathroom while the men's bathroom next to it is not, this is a clear violation of the equality of access principle.
If women are prohibited by law from excluding men from rape crisis centres and domestic violence shelters, because the men identify as women, this has rather specific consequences independent of whether the transwomen (or free riders exploiting self-ID) feel hurt by this. There's a legal challenge right now in Sussex here in the UK to ensure that just
one rape crisis centre is women-only, accepting that transwomen will be welcome at other centres in the same county.
Transwomen athletes are being either included or excluded from women's sports, not because they are necessarily creepy AGPs who want to engage in dick-waving in changing rooms, but in the interests of fairness and safety. Safety is why both rugby associations in the UK banned transwomen from participating in women's rugby this year. Swimming has now banned transwomen from participating in international competitions if they experienced any part of male puberty, after reviewing the evidence.
The question is whether we should deny the rights of one group because of the misbehaviour of another group if the two groups can't be separated. Or should we deny an individual in a group the opportunity to exercise their rights, just because it is a detriment to the group? We believe in universal principles here, don't we? This argument has been made to me in defence of feminism before now.
No, there are more questions than this around the principle of self-ID. Identifying as a member of a group is insufficient grounds to accept the self-identifier into the group in a number of cases in society, especially if one is seeking to do this to gain access to restricted spaces or privileges. One cannot identify as a child if one is an adult, those who pass themselves off as aristocrats, the wealthy, army officers, Vietnam veterans or as ethnic minorities who are found out tend not to fare well.
There are conversely many groups one can identify into quite freely - sexual orientation, for example. In employment and job applications, equality, diversity and inclusion legislation and company policies might allow some to decide to identify as a minority or protected group to help them get shortlisted for an interview - the organisation isn't then obliged to give them the job (I've seen that happen in job presentation/interview panels) or promote them. In other cases that might happen. Doing so doesn't result in explicitly direct harms (there are usually many candidates for good jobs or internal promotion opportunities), while equality legislation also protects others against discrimination, or enables them to mount challenges. If someone declares themselves to be a transwoman when applying for jobs or once in post, then they can do so, just as they can declare themselves to be lesbian, Christian, disabled or any other protected characteristic.
There have been cases where a transwoman took up a position as a women's officer in UK organisations - a sex-specific role that can be restricted to women by law. They lacked a gender recognition certificate so were legally still men. This is a good example of a violation of existing equality legislation. Self-ID as a principle would blow open other legally recognised exceptions, or make it impossible to offer support for one or the other sex, e.g. scholarships.
'Rights' also can be enforced - and this is the other problematic aspect of talking in very vague abstract term about rights. There are proposals to turn 'misgendering' into a hate crime in some jurisdictions. But this criminalises speech, which automatically comes into conflict with freedom of speech, generally enshrined into the constitutions and legal systems of all western societies. There are ways one can avoid using 'preferred pronouns' in direct and written conversations, and this will likely become ostentatiously common to avoid using he/she/they in circumstances where this might be implicitly or explicitly policed.
In the UK, gender critical views have been recognised in a court ruling as a protected belief under equality legislation - so the insistence that 'transwomen are women' cannot be legally compelled from individuals exercising freedom of opinion and speech, for now.
That leaves the classic bathroom/changing room access discussion. Men who identify as women would like to acccess women's toilets and changing rooms, or are allowed to do so in a variety of jurisdictions. They are a minority, whose desires are cast in terms of 'rights', but these conflict with the desires and rights of women, at the very least with a minority of women who may decide to object. Both sides appeal incidentally to the dangers of using the 'wrong' bathrooms - transwomen suggest they are at risk of attack by other men, while women note cases where men claiming to be transwomen have exploited the loophole to engage in voyeurism, exhibitionism/flashing and sexual assault.
The problem with self-ID is that it makes it impossible to define transwomen as a group in such a way as to exclude other men who wish to exploit the new loophole.
In cases where women's toilets are replaced by unisex toilets while men's toilets remain men's toilets, the loophole is extended to every male irrespective of gender identification, while preserving a male-only privileged space.
The logical solution is to ensure greater provision of gender-neutral spaces - multiplying existing gender-neutral disabled facilities - to the extent that there is an actual need to accommodate a greater than negligible minority of transwomen.
In terms of rights, the rights of women, as the numerically greater group who are also the physically weaker and more vulnerable, should logically prevail over the rights of men identifying as women, in cases where these clash. Security personnel and police should not side with transwomen or men simply claiming to be women against the views of women, and transwomen should remove themselves if they are making women uncomfortable with their presence or behaviour. However laws might be changed to accommodate trans identities, then indecent exposure and other laws on sexual offences must still be enforced and should not fall by the wayside. Since transwomen who are trying to pass should be entering cubicles in toilets, there is no good reason for them to expose themselves in such situations anyway.
The most widely reported case concerned a sauna (the Wii Spa case), while there is a legal precedent from *Canada* that women offering waxing services are not obliged to serve men identifying as women (the Yaniv case). The balance should tip towards respecting the desires and rights of women not to have to share intimate spaces with natal males. In many other adjacent cases there are existing mixed-sex options (eg public swimming pools mostly provide mixed hours, and only a limited number of women-only hours or children-only hours).
The question of whether transwomen are women and should compete as women in sports, and thus which changing rooms they should use, at schools or in professional sports, illustrates how these issues converge. There are three levels to the question
1) are they legally women/can they identify as women?,
2) should natal males be competing alongside natal females in sports,
3) should they be sharing changing rooms?
Societies as well as sporting associations are starting to offer very different answers to these three questions, so there are already inconsistencies and contradictions, some of which might be open to legal challenge if self-ID and the view that 'transwomen are women' wins out. One can imagine scenarios where transwomen's legal status 'as women' is recognised (via a self-ID provision or an eased statute) but they still cannot compete in specific women's sports, so the question of whether they can enter women's changing rooms becomes potentially moot.
The biggest area of conflict will be in schools, especially those that encourage trans identifying children to use the changing rooms of their identified gender rather than their natal sex. Since the global trend generally restricts surgical transition to adults, trans girls will still have penises. Especially during puberty and adolescence, it seems to be a very bad idea to encourage or allow such genderswapping of changing rooms, to reduce discomfort first and foremost, both among girls as well as among trans boys who used puberty blockers.
If the trans social contagion bubble pops and deflates, as it might do before long, this will be less of an issue. But it will hurt the overall trans cause if a future generation of teenage boys exploits a self-ID, affirm-your-identity loophole to enter girls' spaces and act like, well, teenage boys. On balance, it is surely better to encourage other teenage boys to be more accepting of gender noncomformity in their own spaces, and to police any bullying, as one sincerely hopes is already the case (with gay teenagers and others). The same goes for teenage girls vis-a-vis trans boys, of course. Rigid and early sorting 'by gender' is probably more harmful in the short and long term, but I'd welcome detailed research on this issue to see how it is affecting
actual children and teenagers.
Gender identity for children and teenagers also potentially conflicts with parental rights or wishes - the balance currently seems too heavily tipped away from parents, and the reported practice in some cases of hiding a new gender identity from parents raises huge red flags. In cases where parental opposition to identifying as trans is considered child abuse, this seems to have gone entirely too far, while there is insufficient attention being paid to safeguarding of children in schools. Children are rightly denied a whole range of rights accorded to adults, while also exploiting workarounds to access materials they're legally prohibited from accessing (eg porn, alcohol, some drugs in some jurisdictions, tobacco, motor vehichles).
Discussing children and teenagers shows how 'universal rights' might be too abstract to properly address the actual permutations arising from this debate. Trans people are much more diverse than the term and debate often suggests; the interests of trans-identifying children and teenagers, especially trans boys, are very different to those of late-transitioning transwomen.