Excuse my ignorance, but I've always felt that sexual attraction has nothing to do, as such, with genitals, but with instinctive attraction. That goes far beyond genitals does it not? If, as a female, you are attracted to other females, then you are a lesbian; if you are attracted to males then you are heterosexual, and if you attracted to either sex then you are bisexual.
Yes, I think this is true. For the most part people are attracted to a package, usually male or female or sometimes both, though IME the latter isn't always the same phenomena.
As far as trans persons though, I actually don't think they fit into this paradigm of attraction with a category of their own. There is no such thing as a trans orientation, because its not a category of sexual reproduction. And it's not really about some intellectual set of categories based on genital configuration, it's about the very old lizard bits of your brain that intuit who you should mate with.
If someone is attracted to a trans individual, it will be in terms of the way their body and mind reads or interprets the signals they get from that person.
So if a gay man who is exclusively attracted to men on occasion is attracted to a trans man, it's probably because his brain at some pretty basic level is interpreting that person as a man - even if he knows that the trans man actually has a vagina.
I don't think it would be accurate to call that person bi-sexual, and that's not about some sort of problem with bi-sexual people. It's just recognising that when you have a person who has changed their body in such a way as to mimic the other sex, it confuses the picture in terms of how our perception functions, it's not changing the orientation or revealing something new about it. (And in a lot of cases I think what actually happens is that confusion creates a situation where many people won't be attracted to the trans person because there are mixed signals about whether the are male or female.)
This could potentially create a situation where, for example, a gay man had a heterosexual relationship. But honestly, it's not like human sexuality hasn't always been a little inscrutable and prone to weirdness.