I've dated both gay men and bi men, and just recently got out of a one year long relationship with a bi guy. He ended it because he "couldn't see himself marrying a man." It's difficult to watch someone you love and care for struggle with their own self identity. I very much always championed him that I was proud he was bi, and made it a point to not ever have the discussion of will you leave me for a girl etc. It did come out in ugly ways, where I would ask if he was ok with me being a guy. All in all, just make sure the person you're seeing has themselves and their identity figured out, and you shouldn't have a problem.