This is a really complicated issue, one I've thought about a bit since I'm a non-transitioning (currently, anyway) FtM and my girlfriend is nonbinary. We look like a lesbian couple, and there's not a lot I can do about that, since I don't pass for ->-bleeped-<-. It doesn't bother her since she's not straight, but I can see where it would be incredibly confusing for someone who is.
If you're attracted to men and male bodies, you're not a lesbian - because the definition of "lesbian" is "a woman who is exclusively attracted to other women." So your question shouldn't be "am I a lesbian?" but rather "am I straight?" - also keep in mind that sexuality isn't written in stone, and the labels that fit you best may change over the course of your life, as you learn new terms and learn more about yourself.
I don't know if you're familiar with the concept of romantic attraction vs. sexual attraction, but that might explain what's going on here. Basically, everyone has both a romantic orientation (heteroromantic, homoromantic, biromantic, panromantic, aromantic, etc.) and a sexual orientation (heterosexual/straight, homoromantic/gay/lesbian, bisexual, pansexual, asexual, etc.). Sometimes they match and sometimes they don't (which can be part of how you get "exceptions," like a guy who's been in heterosexual relationships all his life having that one boyfriend). My best guess is that you're heterosexual and heteroromantic, and because your SO is a man in a female body, it's causing conflict between your romantic and sexual attractions to him. But the point is, it's entirely possible to be 100% straight and still be in a relationship with someone who's trans and pre-transition.
As far as sex goes and your SO's body goes... all I can really recommend is talking about it with him. Chances are he's worried about it too. The best thing you can do is be honest.