I dunno, man. I consider myself pretty fucking straight. I'm pretty sure gay men aren't going to be attracted to a transwoman because she decided to keep her dick. But hey, I'm not a gay man, so what would I know? The way I see it, if a person identifies as a woman, looks very much like a woman, and I find them attractive, that's all I really need.
Though, yes, I would say sexuality can be a pretty big spectrum of different factors of attraction and that someone can be a small degree of bisexual. But I wouldn't agree that dating and having sexy times with a pre-op transgender person somehow makes a person not straight.
Granted, that's not a huge deal either way. Or at least, it shouldn't be. But everyone knows how tons of people just gotta know how other consenting adults go about their private business. Because reasons/religion/ick factor.