I think Dan Savage is right when he says a lot of straight men are curious about cock--after all, they've spent their whole lives obsessing over their own. But straight men are only interested in women. The idea of having sex with a man is repulsive to them. So, if they want to play with a cock that isn't their own, that leaves them with transwomen who haven't had sexual reassignment surgery.
I also see what I believe to be a desire to try something exotic. A lot of men enjoy variety, and for the more adventurous, that includes transwomen. Again, if they are straight, sex with a man is not an option.
And just as straight men are repulsed at the thought of having sex with men, gay men are repulsed at the thought of having sex with women. Gay men have no interest in transwomen.
Of course, there are bisexual/pansexual people out there who don't really care about these distinctions, and have sex with whoever they feel like: male, female, FtM or MtF.