I'm having a hard time seeing how a heterosexual Christian takes it upon themselves to terrorize a gay couple in order to split them apart, is at all comparable to transwoman who goes about her day-to-day life causing no harm or grief to anyone.
In both examples someone was making a unilateral decision to screw around with someone else's life choices and sexual existence because they decided they knew better and that the other person's desires could be ignored because they didn't agree with the basis of them.
Knowing your partner's intimate thoughts and feelings would certainly help you make an informed decision about whether you commit to them, but I doubt you'd say this gives you a right to snoop through their private diaries and emails. That must mean your right to know is balanced against another person's right to privacy.
Someone having been born male when they're representing themselves as a female or vice versa is a pretty big thing. It's a very unusual thing, and wouldn't you agree that the majority of people, both men and women, who are straight and your average sort of person... would not knowingly enter into a relationship with someone if they knew that about them?
That is undeniable, I think. If you could poll every straight male in the US for instance, and ask them whether they would have a sexual relationship with a woman who was born male and had their penis surgically removed... what percentage do you think would say "no"? I'm thinking the percentage would be in the high 90's. I think such a widely and strongly held desire cannot be easily dismissed or treated as trivial.
As for the sorts of things a person would be likely to discover in someone else's diary? Well, you'd have much lower percentages saying categorically "no" for that sort of thing. Most of what would be revealed in there is that the person has had a lot of drama and trauma in their life, might be more clingy/obsessive/stalkery than they appear... maybe has had more sex partners than you realized... etc.
I think it's much easier to categorize that sort of stuff as "normal expected variation among people in our modern age, that's life" than someone being born one gender and having serious surgery, hormone treatments, etc. to change that gender. Again, that's a pretty unusual and extreme thing far outside the experience of most people who are out dating, and far outside what they expect as a possible experience while doing so.
You definitely need to know whether your partner has an STDs, for your own safety and the safety of future partners. You need to know whether your partner has food allergies before you cook for them, for their own safety. You need to know about one another's fertility if you want to start a family.
Well the problem is that it's quite common for people to get pretty far into a relationship without the subject of kids ever coming up. These days, people will have been together, intimate, and deeply intertwined in their lives (living together, etc.) for a long period before any discussion of kids ever comes up, in many many cases. It may be that neither partner had even given it any thought, but as they age and start to feel "this is the person I want to be with for the rest of my life" a natural outgrowth of those life developments can be that they start thinking they'd like to start a family with that person.
So the problem with how you're framing this is that you cannot know whether the person will need that information from the point of view of starting a family. Even if they say to you that they don't ever want to have kids, it may just be youth talking and they may change their tune dramatically later on that point. It happens plenty often that someone talks a big game about "I never want to have kids" but finds that as their biological clock is ticking and as they start to get older, they feel differently.
This is why I think people have to be made aware of such an important matter (whether their partner can be a reproductive mate, which is the default assumption) or not. That's pretty huge. If someone is sterile and knows it, or if someone isn't a born female or male and knows it, I think they're obligated to inform any potential partner before any physical intimacy takes place. That's my view.
But you don't need to know everything. You don't need to know whether your partner has ever had cosmetic surgery, if they used to be fat, used to be anorexic, if they've ever taken ecstasy, ever had a same-sex partner, whether they are a virgin, whether they were ever raped, whether they have Jewish ancestry, whether they have Kleinfelter syndrome, or a complete insensitivity to androgens.
I know it's important to a lot of Jewish people to have Jewish children with a Jewish partner. It's often even more important to them that their children find a Jewish partner. They're a small ethnic group and they're interested in survival, so I think that's understandable but I would think it was understandable even if they were the largest group on the planet. It's their right to value their genetics and wish to keep their family consistent and connected in that way across the generations.
That's a right I would say extends to anyone from any group. If it's important to them, I don't think it matters if we as onlookers agree that it should be important to them, or if we think or don't think that they have good reasons for finding it important. It's their life, and to the extent reasonably possible, I would think we'd want to be a society that honors peoples' deeply held beliefs and choices.