I think promiscuity (healthyresponsible)becomes a problem basically after
conflicting human emotional unexpectedly show up in either participant,
and consequently transfigures a former informal "no strings" contractual
coupling into something more personal and even damaging.
I have been there, I.D. We go into these relationships with eyes wide open, though, so if both parties are honest, we only have ourselves to blame for continuing on and getting hurt.