The problem with vixen is that it also has a bitchy connotation. Also, foxes aren't especially promiscuous. Grey foxes pair bond for life, and red foxes are serial monogamous (one partner each year). Not to mention, they only fuck like 1 or 2 weeks a year.
And perhaps that leads to the word I was looking for... promiscuous. It comes from Latin and origin means across types, a variety, non-discriminatory based on group.
It does have a negative connotation that too many cheapens the value of each, but I have never found that to be the case with sex.