You have to be comfortable. If you aren't comfortable you aren't comfortable. You may wish to delve into yourself to determine why you are uncomfortable, but even if you land upon the reason, the feelings aren't just going to magically evaporate. Your reason may be irrational, it may be a combination of intuitions who have yet to put a finger on, or you may have a perfectly valid reason for feeling this way. From where I sit, its irrelevant. I don't question people's sexual preferences and I don't pressure them to change to suit other people, or what is popular. Sexuality is too personal and too deeply wrought in our psyches to be forced or coerced or politicized. This is about having fun and sharing pleasure with those who want to share it with us. If you are uncomfortable, you aren't going to be having fun and its going to suck for everybody involved. You may, upon reflection, find your feelings about this change over time. And that's okay, too. We all learn and grow as we cogitate upon new experiences and become more comfortable with the unknown. That said, I will play with couples where the man is bisexual because I'm upfront that I am straight and M/M play is not on the table. I have yet to meet a bi man who was anything but respectful of my boundaries.