Now this is a good question. I hope some day I'll be able to answer it.
So far I've come to the tentative conclusion that in terms of sexuality I'm a straight woman. That is, my ideal sexual relationship would involve me, in a female body, with someone else whose body was the opposite sex to mine. I can make some compromises, like being in a heterosexual relationship as a 'male', or being in a lesbian relationship, but I think that's about as far as I can go. Mostly it's about anatomy, though, and not identity.
Relating this to the question here, I think I would have no problems with an FtM, although I think I'd have to become a bit more feminine as his transition progressed. Likewise, I'm pretty sure a post-op MtF partner would be no problem for me; in this case, coping with her early stages of transition might require some adjustments to my own gender.
Anyway, it's all highly hypothetical and I don't intend to make an empirical study.

Nfr