Hopefully nobody will mind me chipping in my two cents, even if they are slightly overdue. I'm a genetic male and consider myself straight (should anyone be wondering: the reason I joined Susan's is because of my girlfriend, who is a post-op m2f).
Most men (the large majority) don't look for a t-girl at all and stick with genetic girls. This is because they either don't know, or because their fragile ego's are too easily overwhelmed and can't handle it (ever notice that sometimes when you tell a man you're m2f, it suddenly becomes about him and how you 'reflect' on him, and not about you?).
There are a few men who don't actively look for a t-girl over a genetic girl, but would have no objections dating one or having a serious relationship with one. To them, it makes no difference if a girl is a genetic female or a t-girl. I fit in this category. They're both women, and if you're into women, what's the problem?
Then there are men who actively pursue t-girls over genetic girls. Some of them (albeit few) do so because they are looking for certain qualities which genetic females don't possess in equal measure, the femininity (sp?) thing, for example. One of the things I like the most about my girlfriend (apart from her eyes) is that she behaves way more feminine than any genetic girl I've ever met. I was drawn to her because of that, so I can understand if some men are actively looking for those qualities (I wasn't).
The other kind of man who actively pursues t-girls over genetic girls is the one who does so for the
wrong reasons. They aren't interested in the person herself at all, they are only interested in sex with a t-girl exactly because she is a t-girl . The fact she's a transsexual is objectified (=they don't look at t-girls as people, but like objects that are there to fill their urges). It is made into a fetish, which they want to satisfy. They have no interest whatsoever in the people themselves, and (ab)use them to get their own way. They aren't interested in a genuine, real relationship. Their family and friends don't know about their "fetish" and they want to keep it hidden from their surroundings. They are lavish with words but will never back them up. These are the ->-bleeped-<-s. Unfortunately it seems most men who approach t-girls fall into this category

. Sometimes it can be difficult to discern if they're being genuine or not.
I'm not saying every men can be categorised as such, but from what my girlfriend told me I think many can.