Once again a lot of this comes back to that horrible error of judgement where people make the assumption that trans women are men, rather than having been women all along who where forced to hide themselves. It is the source of most of the misinformation about trans women. Straight men sit around and think "hmm why would a dude want to be a girl?" and the only answer said straight guy can come up with is "to have sex!" because to most men, conciouslly or not they belive that women are primarily a sexual object and so anyone wanting to be one must want to be a sexual object.
This view is deeply tied to the ages old theory that trans women are just really really really gay guys who want to be girls to have sex with more guys because people confuse sexuality and gender such that gay men are seen as feminine, really its all a horrible toxic mess.
As to the odd obsession some men have with trans women I think it comes in two parts, the first is fear and concern in short the idea that they may find someone they see as male attractive makes they afraid they might be gay. This is also usually the source of the idea that we are traps/liars. So on that one end there misinformation makes them feel fear and concern over there own sexuality.
The other side of it is those who get over that homophobic fear and who are obessed with the idea of having sex with a trans woman, not because she is a woman but because he sees her as a man. Men seem to have some sever sexual power trips, Obsessed with having sex in the first place and as time goes on, they often seem to want the harder to obtain sexual conquests, strong willed women, virgins, lesbians, those who they see as harder to get into bed or who would be seen as harder to control seem to be a obsession. For some the idea of sex with a trans woman is equated with sex with a man and because in there deluded, misogynistic brains men are stronger than women transwomen are deeply into that catagory of greater sexual conquest.
Or thats my theory, you can read about it in my new book "Men, WTF is wrong with them and do we even really need them anymore" sure to be on your kindle and at your local book stores next summer :-P
Serena, who is probably not a man hating lesbian but who knows.