"Put simply, we are proposing a shift from asking Why engage in SSB?' to Why not?'

Put simply, Sexually transmitted disease. The more "sex" we have more likely to be infected, regardless of who you have sex with.

Is it possible humans ability to thrive was to mitigate as much as possible STDs? And Disease in general? through religion? Marriage et al.

The human species probably did/was as promiscuous as other species, then we used our ability to think about what denominator was causing people to die, and created heuristics, and embedded them in our religions.