I’m just going to pop in and with all respect say a couple of things:
There have been multiple cultures with more than 2 genders, and as others have pointed out, sexes are more complicated than xx/xy.
Interjecting a fun fact, did you know male calicos are so rare because they have xxy chromosomes? In calicos the color pattern is tied to the chromosomes.
To quote a favorite YouTuber of mine, nobody goes “technically, you aren’t the biological parents” to foster parents, and instead we respect their responsibility. I can’t remember the exact wording, so I might be making it sound a bit more strawmanny, but it’s the best analogy I’ve found. And it doesn’t erase that the biological parents are out there and maybe something medical could come up or something, but foster parents are raising their kids as parents and living that life. Someone who transitions is living a different life, and anything medical is their business, the same way we wouldn’t butt in on the biology of foster parents.
Also, if someone physically transitions (beyond physical presentation/clothing/chest binding/etc), a lot of their biology changes. I don’t know how true it is, but I’ve heard people say that they started having certain things be more in common medically with the gender they transitioned to. Like supposedly, their doctors would treat certain things much closer to the way they would in the gender they transitioned to. I’ll just leave that as a point of research, but regardless there are obvious and super visible physical changes.
It doesn’t override your sex. I wouldn’t argue (and I don’t think people should argue) that it totally erases it, even if you go the whole nine miles with a physical transition. But really, if someone did that then they’ve taken on the responsibility of tackling their dysphoria and changing their body and presenting how they want. That’s worth respect. And even if they aren’t physically transitioning, they deserve the respect. They’re confronting their feelings and how they want to present and a level of dysphoria. It’s a simple thing to address someone by their desired pronouns (anyone who’s instantly angry if they’re misgendered on accident probably has anger issues anyway, no disrespect to my fellow transfolk, that stereotype is a strawman- and anyway, intentionally misgendering people would make anyone look silly).
Personally I just accept any pronouns because I’m not expecting to keep everyone, or even my boyfriend to keep up to date with my desired pronouns at any given moment. Sometimes I can be totally convinced that I want to transition and a couple hours later want to embrace my assigned gender, and sometimes I feel like I’m agender, and sometimes I oscillate over minutes. As for how I recognise that or where I draw the lines- or why I am this way- well, that’s honestly too personal for me to share here.
Ultimately, people don’t stick with this for attention (and of course there are fringe cases, but there are with everything and the issue isn’t trans-ness itself, let’s just not). It isn’t easy, life as a transperson comes with a lot of disadvantages due to how society responds, general identity confusion and medical care for it, and the dysphoria that leads to this is not a choice.
even if I look or present myself in a "feminine" way, I'm still a trans guy. there's no debating there.
I usually present very androgynously, though I tend to look more traditionally "feminine" than "masculine". I actually take some pride in dressing effeminately and then wearing my he/him pin hehe 
