LOL - Why do you hope you're not? If you are, it just means you doubled your pool of potential sex partners (real, online, or fantasy). You don't have to start spending half your weekends in gay bars. :-)
I have to tell you--the first time I ever had an online Dom, he would ask me if I was into this or that (e.g. hair pulling, being called his "bitch," etc.) and I would say no. We would get into a session, and when I was all sweaty and delirious, he would spring it on me. Knowing that he was doing it even though I asked him not to, plus seeing the whole thing through the lens of being so turned on, made me totally change my mind about those things.
I guess what I'm saying is that as you explore being submissive, you'll find out that things you thought you *don't* like can be an even more powerful turn on than things you *do* like. When something's a little bit replusive to you (I'm not talking about hard limits, here, just little things), but you do it anyway to please your Mistress, your whole view can change.
Regarding you being bi or not--I think you know you're not really attracted to men. That's fine. A lot of people fantasize or play about being forced to engage with other people in sex acts, whether they're attracted to them or not. You just want to be humliated, any way your Mistress wants it. If that's another guy, then so be it. It's the humliation that turns you on, not the man.
One final comment--please be careful about thinking you should never say no. It's okay to not like certain things, to find out that you don't like things you thought you would like, and to find out you do like things you thought you wouldn't like.