It's all about your personal boundaries and what a) you can deal with, and b) expect your partner to deal with.
I, for one, could NEVER be in an open relationship. As much as I'd want to trust my partner, I could never deal with the idea of them with someone else. BUT, that said, I know of a few couples that are totally ok with it and have been together for a long period of time. It's just not for me, and my partner would have to agree with me. BUT if that's what works for you, the more power to you. You and I will never date. (you meaning the generic 'whoever you are' you)
Celebrate Life
Experience is what you get when you didn't get what you wanted.
- Randy Pausch