I'll answer from personal experience. I've only started thinking about polarity since I caught up on Steve's posts about it this week, but I see myself as light-oriented if not yet consciously committed. My fiancée is preparing for a life of service, possibly becoming an Interfaith minister. So I think we mostly describe the relationship you're asking about. (We're not married yet, but we've been together for nearly six years now...)
We have an explicitly open relationship. Neither of us has any reservations about the other having sex with anyone (responsibly and safely, of course). We want each other to be happy. We do have our share of fear; we don't want lose each other. But we're not worried that sex outside the relationship will hurt it.
Honestly, I have trouble understanding the motivation behind a completely monogamous relationship. I want her to be as happy as she can be, and to make others happy in the process. I can't see why it should make me jealous or unhappy that she would.
*shrug*