But it seems to me you're saying that if you're with a trans person, it should be not because they're trans, but in spite of it.
That's exactly what I'm saying.
Trans people just want to be accepted as their chosen gender. A trans woman wants you to see her on the street and think "wow, that chick has really awesome shoes. I wonder where she got them?" And not "wow, that trans lady looks really convincing. I wonder if she's had surgery."
Or do you think having absolutely no preference towards your partner's genitals, the thing you're having the actual sex with, is a common thing?
Trans people generally find partners who are bi or pan, i.e. people who are far less likely to care what kind of genitalia they have. It's rare for homosexuals and heterosexuals to remain attracted to someone when they realize they're trans. It often leads to shock and even violence, which really sucks.
Bisexuality / pansexuality are pretty common, and getting more so as each Gen Y'er hits puberty.
I guess I'll never understand it, and it's not fair to expect you to keep explaining it when I'm just not getting it.
I'll start with a small digression. In our bi/pan-group, we've been discussing the term "bisexual." A lot of bi people choose not to use that term because they feel it emphasizes sex when what they really care about is relationships. Some people have started using the term "biromantic" to indicate that they're interested in forming long term, loving relationships with either gender. Others just say "bi" because it encompasses both sexuality and romance.
The rest of this isn't as binary as I'm going to make it, but it should convey the basic idea.
For simplicity, I'll stick to "straight." "Gay" is the same but replace "opposite" with "same." There are two kinds of attraction: romantic and sexual. I'll use the terms heteroromantic and heterosexual to distinguish. So heteroromantic means you only fall in love with people of the opposite gender as you. Heterosexual means you only are sexually attracted to people of the opposite sex as you.
If you have a penis, heterosexual means you only want to put your penis in vaginas. You may or may not care if that vagina is attached to the body of someone who identifies as male.
If you're a man, heteroromantic means you only have relationships with people who identify as female.
Now, for most people, these orientations line up. When they don't, it tends to be that one or the other is not hetero/homo, but rather bi. I've never heard of someone who only falls in love with women and only enjoys sex with penises. I'm not saying it's not possible, but I've never heard of it. But supposing that such a person did exist, that would be an "orienation" and not a "fetish."
Gender and gender identity is central to how people see themselves. It's the first thing anybody notices about you, and it affects how you're treated more than any other factor. Everybody has a gender, or a specific lack of gender (genderqueer or gender neutral, for example). It's the main box that everyone gets slotted into. So that, unlike being fat or being crippled, is what makes it different from other fetishes. And for all its ubiquity, it's one of the least understood things out there. "Why can't you just be a woman? You were born with a vagina, that makes you a woman. Why can't you just accept that? If God wanted you to be a man, he would have given you a penis." No one says "why can't you just, you know, not be handicapped?"
In this thread, we've been using the word "fetish" as a shorthand for "sexual fetish." Fetishes aren't about relationships. That's not saying that people with a certain fetish can't seek relationships with other who share that fetish or its complement, indeed they usually do. But the fetish is not "the relationship."
ALSO, because all that wasn't complicated enough... most trans people feel a lot of anxiety and dysmorphia about their genitalia. Because they're the "wrong ones" and they don't "fit right," they often feel repulsion towards them. Like, imagine waking up one day with crab claws instead of hands. It's kinda like that. Crab claws are perfectly great and useful... if you're a crab. If you're not a crab, they're awkward and repulsive.
Sooo... because they don't have good self-esteem about their penis or vagina, it feels really gross to have someone say "Hi lady, nice to meet you. I like your penis. Wanna fuck?" Kinda like... "Hi person, nice to meet you. Your crab claws are cool. Can you open this oyster for me?"