The slippery slope argument isn't a fallacy if the slope is, in fact, slippery. Before X is legitimized, those who say X will lead to Y are laughed at for not noticing X is totally different than Y. After X is legitimized, however, the same people who previously claimed this X is totally different than Y usually also want to legitimize Y, claiming it is very clear X and Y are actually really similar -- precisely the slippery slope argument they laughed at with their opponents as long as their goal was to legitimize X.
I'd like some verifiable examples of this claim, please.
We've seen this happen, for example, with divorce.
Have we? Who is "we"? What nations and cultures in what time periods?
Those who claimed easy divorce will increase illegitimacy were laughed at, told clearly these two things are totally different.
Were they? Where? Did anyone actually use that as an argument against divorce? Some nations have had legalized divorce for centuries, others only for decades. Cultural concepts of marriage and divorce vary considerably from place to place.
Plus, your claimed link between divorce and increased rates of illegitimacy doesn't bear out in all cultures. For example, Japan has had legal divorce since the 1600's, but they didn't have huge rates of illegitimate childbirth.
I suspect that modern increased rates of illegitimate childbirth is more a product of welfare payments, less stigma, and other modern changes in society that make it easier for women to raise children without a partner.
We've also seen this happen with civil unions. At the time those who supported it claimed it is totally different than marriage, and thus only people who are bigots will claim that it will lead to gay marriage. Needless to say the minute civil unions became legalized the fight to legalize gay marriage -- naturally enough, now considered very similar to civil unions -- started. Once again the "slippery slope" folks were 100% correct.
In many places civil unions
are considerably different from marriage, from both a legal and cultural standpoint. Although the exact nature of what counts as a civil union varies depending on where you live.
We might well see this with polygamy: first we are told there is no relation, and marrying two men is something totally different than one man marrying two women; I myself was often told this on this forum. But if and when gay marriage is normalized, why, then one can bet -- judging from previous experience -- that suddenly it will become "obvious" to everybody except "bigots" (or so the proponents of polygamy will claim) that surely, if two persons can marry, this is suddenly very similar to three people marrying.
If three or more persons wish to be joined together in a group marriage, what's wrong with that? I'd support a system of polygamy where all partners have equal standing.
But polygamy definetly isn't the same as monogamy. Allowing same-sex couples to get married won't change the arguments for polygamy in the slightest.
If your argument were valid, shouldn't all the cultures that supported polygamy in the past have been more willing to accept gay marriage? After all, you're arguing that allowing one makes the other seem more acceptable. This argument works both ways, but no such trend has ever been observed in history.
But since it applies equally well to any sort of X whatever, then there is no way to stop polygamy, prostitution, sadomasochism (wait -- that's already de facto legal at least), necrophilia (the corpse isn't a person, so who is hurt?), incest (between adults), or any of the numerous other psychopatia sexualis from being normalized, as well.
Incest can result in harmful effects to any potential offspring. Plus, there's a matter of potential psychological harm in situations where one person may feel pressured to have intercourse with a close relative.
Defiling a corpse can cause emotional distress to the deceased's relatives. Plus a corpse isn't a consenting anything, let alone a consenting adult, so that argument doesn't apply. (More a case of vandalism of someone else's property?)
Fun fact, there's no federal law against necrophilia in the USA, and some states have no specific law against it. This results in some interesting court cases.
For example. 
As for prostitution, is it necessarily a bad thing? Over here we've legalized it and it hasn't caused any discernible moral decline. Up in Queensland, a brothel called Daily Planet made headlines in 2003 when it became one of the first legal brothels listed on the stock exchange (in the form of a property trust). Apparently it's been in business since 1975, long before it was legalized.
