This is a really interesting thread. I’m bisexual (I’m married to a man and therefore most of this is moot now) and genitals matter to me too. I think that sexual attraction goes a lot deeper than just genitals, and yes, it’s biological. Hormones, pheromones etc all play a part. And I think that’s why I’ve yet to be sexually attracted to someone who is trans; plus no matter what they say, it’s pretty obvious which sex they are (yes, a single photo can hide it, but generally being in the presence of a person it becomes clear).
I’m trying to pinpoint exactly what it is and it is difficult to articulate. For example, my sexual attraction to men is a lot more narrowly defined than my attraction to women - I prefer masculine looking men (physically; their expression can vary - see later in post) but I can be attracted to feminine and masculine looking women (with a more defined lean towards the latter) but I would not be attracted to a transman, particularly one who had surgical changes (and this is also where sex characteristics matter: I’m attracted to women with breasts and vaginas and men with penises, neo “either” doesn’t cut it).
And of course the mind comes into play too because while we do have physical sexual attraction and that is important, that isn’t the only factor we build relationships upon, which I think for me is where it’s more about preference within my sexuality. For example, I would love and be attracted to a woman who had a masectomy for medical reasons despite the secondary sex characteristic I’m attracted to being gone (and that would be a sexual attraction I would miss) - to be crude, she’d still have a vagina; but I’d not be sexually attracted to a transman who felt they weren’t biologically female because the signals MY body would be receiving from them would tell me they are, and those signals matter. In the same way, I find women wearing masculine clothes are often very sexy, and sometimes men with certain feminine ways about them but who are obviously still men (think new romantic types, Marc Bolan etc or even Tim Curry in Rocky Horror) are also sexy. Interestingly, I used to find old school Eddie Izzard attractive, but the more he has tried to emulate femininity the more I went off him (and it was a physical reaction) until it vanished altogether.
Intelligence also features heavily in my sexuality. Many a person has opened their mouth and talked bollocks, and consequently my physical sexual attraction to them vanished in a heartbeat, never to return, while on the flip side, a single characteristic such as a certain voice (hello, Alan Rickman) can send me into orbit and other aspects I might care about suddenly don’t matter.
I would never say I would never be sexually attracted to a certain type of person (e.g trans) but I can say it hasn’t happened yet (frequently, I have the opposite reaction; it reminds me a little bit of an uncanny valley response, for those who know their psychology).
I don’t even know if that makes sense, but that’s how my sexual attraction works.