I don't think this is any sort of polygamy, but I do maintain a "friends with benefits" relationship with a guy I've known since high school (where we were the "are they dating or not?" couple). We don't see one another romantically if either of us is engaged in a closed relationship... but if we are both single and just want to have a romantic partner available to hang out with, make out with... yes.
We realize that we're not compatible for a long term romantic relationship, but we're very good friends.
I also have a female friend that won't admit she's bisexual (she's just going through "college experimentation"
), and we mess around once in a while... but again, only if neither of us are in another relationship at the time.