People have a right to choose their own life-style. Some people are into "swinging" and think it's just fine to swap intimate partners. If both members if a couple want to do that, then that's their business. But it's not what you signed on for. What your boyfriend is doing is the behavior of someone who doesn't value exclusivity in a sexual relationship. You do. It's a good value that most human beings share. I don't care whether or not he feels "romantic" towards this gal. Some of the advice above will do nothing to advance your happiness. Stick to your values, which make you who you are. You have every right to defend them.

You might need to seriously think about whether or not your boyfriend and you are compatible. First, see if he's willing to get on the same page with you over a fundamental value.