Hi Trixie. I'm FtM. With respect to your orientation, I've heard a lot of lesbians express similar concerns about their partners transitioning (whether that makes them straight). I don't have a clearcut answer to it. You would present in public as a straight couple, which I could see being bothersome to someone used to the lesbian community. Also, some lesbians are too narrow-minded to accept a relationship between someone who identifies as lesbian and a guy, be he FtM or bio. I wish I could offer more words of comfort there. I've seen some of my lesbian acquaintances distance themselves from me since my transition. True friends have stayed friends.
I don't think many of us are 100% one end of the Kinsey scale. It's a scale rather than distinct categories for a reason. Lots of us fall somewhere in the middle, but most of us don't explore the small fragment of attraction other than our main for many reasons. If we identify as straight, we don't socialize with as many gay people, so that side may go unexplored. If we identify as gay or lesbian, our social circle is often composed of mostly gays and lesbians. And you just get used to a certain "kind" of dating, interacting, etc. It's easier to stay within patterns.
All that to say, that regardless of how people identify, there are exceptions to usual patterns of attraction and you are definitely not the only person to experience it.
As to the mechanics of sex, it will depend on whether he's had any surgeries or not. If he's had chest surgery, he may be just fine with nipple play. Or they may not have much sensation. The best person to answer that is him. If he has had no bottom surgery, and you two want penetrative sex, he'll have to use a prosthetic, or toy. If he has had bottom surgery, it doesn't work exactly the same as a bio male penis (would need implants or pumps). Sex with women is probably easier because you have the experience, know signals and patterns, but with a little discussion, it shouldn't be too intimidating with him. Odds are he feels as nervous as you about it.
Dennis