Personally, being a femboy myself, I don't get it at all. I could see it being interesting to genderfluid people or people who are experimenting with gender and I know there are transgender people who'd obviously fantasize about just waking up in the body of the opposite sex one day but they usually aren't sexual and are usually born from the frustrations of, y'know, being transgender.
Any FtM stuff I see mostly has Sub/Dom elements. If the viewer (you know what I mean, the main character stand-in placement for the viewer) is supposed to be getting turned into a girl it seems to me that it has more to do with escaping traditional masculine roles in an exaggerated way* or it's a way of dealing with their own homophobia (As in if I'm forcibly turned into a bimbo then I have an excuse to suck cock, also it's technically straight now.) But I've seen a few occasional ones where the viewer turns their childhood bully or asshole boss into a girl and fucks them, obviously a domination fantasy type thing.
Honestly it mostly seems to me that their's an underlying theme of objectifying women in these fantasies that upsets me so much. If men want to be bottom fuck sluts with their assholes constantly plugged then they should be allowed to feel comfortable with that, regardless of what all the submissive women who want to keep their positions as bottoms claim. (That last bit was a joke; I'm a funny guy.)
Ugh, I don't like talking about trans stuff because I'm afraid I'll offend someone (apologies in advance), and this is all just conjecture formed from my opinion, but I am so sick of every trappy guy being instantly treated like they were born to service men and they all have to be "taught" to be "girls" (looking at you Delilah). I'm also probably too mad and tired to be ranting about my opinions on the internets right now but I'm doing it anyways.
*Just an excerpt from Laura's Playground (A weirdly named site for trans-folk) about why men crossdress. Link to page
I know Crossdressing isn't quite the same as MtF but I think it's the same underlying principal.
Sorry for this wall of text :S
Every person is different, so they reasons are.
But I don't want to be a woman to not be gay (in fact I mostly prefear women or femenine persons, so that make me gynosexual I guess).