In the past, I resist the idea of being in an open relationship. Now, I'm more open to this idea. I've been in several LTR, and what I've noticed is that after several years, the intensity of sex becomes less and less, sometimes non existent. I know friends who are in LTR relationship that don't even have sex at all. They have the companionship but I think they are just having solo sex and not with each others. That is something I would not want.
I've been in threesomes, but not when I was in a relationship. I think it's fun as long as we are all into each other. It spices things up. When opportunity arise, I would definitely go for it.