We sometimes add in a guy friend as a fourth (or James and I see him as our third) and even though we only see him every few months and it’s only for sex, we really like each other as people, and as sex partners and it’s a great dynamic. It’s really sexy, but beyond that, it just feels really natural and right. I’ve been reading up about how some non-human primates use sex to bond and build community. Female bonobos have sex with other females as often as they do males because they use it in this way — to build a community in a way that just feels good.