Both of the long-term relationships in my life were theoretically open. The first one was more open in practice than the second one. We each occasionally participated in extracurricular activities, as it were. But we had pretty firm rules. No secrets was the biggest rule. If we were going to be sexual with a friend, we needed to tell each other about it.

That worked pretty well for us. We never had a threesome, and I'm not sure why other than I guess we just never wanted to.

When my partner died and I fell in love with somebody else, we set up similar rules about clear communication and no secrets. Then we pretty much ended up monogamous anyway. Except ... we DID have a couple threesomes, and even a foursome once. That DID become awkward, actually, and we didn't repeat the experience.

Anyhow, nice discussion starter. :-)