If you're genuinely confused by the concept, I can understand that. I can try to help you make sense out of where I'm coming from, at least -- with the caveat, of course, that I'm only speaking for myself as far as how my own sexuality operates.

For me, an integral desire for sexual intimacy and sexual pleasure with somebody else is to be with somebody whose body and anatomy (sexually) works very similarly to mine. That includes (but is not limited to) ejaculation, erections, hormones, etc.

To my knowledge, when trans men have gender-reassignment surgery, there is currently no scientific way to give them the ability to produce sperm or to construct a fully-functional penis. Will science eventually advance to that point? Who knows. But that isn't the reality, right now.

This doesn't, of course, mean that I'm going to desire (let alone try to coerce) sexual relations with every man whom I find attractive. He needs to be homosexual, as well. And there will be other traits of mutual compatibility that either of us would be looking for. But, yes, genitalia is a dealbreaker for me.

Obviously, this isn't the case for all cisgender people -- regardless of whether they're heterosexual, homosexual, or bisexual. And that's great. Every person should be able to get what they need from a sexual/romantic relationship.

I can absolutely find a trans man or a trans woman attractive on the outside, without knowing how their body works. But that's all it is: initial surface-level attraction. Upon getting to know them better, I can also value their positive character traits that would help ultimately develop a strong friendship with them. But strong friendships don't necessitate sexual companionship.

I'm also not going to attempt to try to speak on behalf of cisgender heterosexual women who desire only cisgender heterosexual men, cisgender heterosexual men who desire only cisgender heterosexual women, or cisgender homosexual women who desire only other lesbians. Nor am I going to try to speak on behalf of cisgender bisexual people who might happen to desire a mate with birth-assigned genitalia. Their experience is their experience. When ANYBODY is confident in their sexual desires, we need to believe them...as long as they aren't causing harm to another person.

Outside of romance: I'd be totally comfortable changing clothes or showering in front of a transgender person regardless of their sex. And I oppose discriminatory anti-trans laws. I'm gradually educating myself, to the best of my ability, about systemic barriers faced by transgender people (e.g. lack of quality health care access), so I can be part of calling upon our lawmakers to make chose changes.

But my romantic life, as a cisgender gay male, is NOT systemic...and not up for debate. I won't be gaslighted about that -- by anybody.