The distribution of oxytocin and arginine vasopressin receptors in the human brain reveals that we are a monogamous species. But is that socially monogamous or sexually monogamous? The large size of the male testicles and shape of the penis suggest that we evolved when females would have multiple sexual partners and it was a "let the best sperm win" all-out competition. We have socially monogamous brains but sexually promiscuous genitals.
Adding testosterone to the mix is like having a wild card in poker--anything can happen. Testosterone is in conflict with the bonding effects of oxytocin (literally--testosterone inhibits the uptake of oxytocin by its receptor). It motivates men and women, but mostly men who have five to ten times more testosterone than the fairer sex, to seek more sexual partners (and to be single-minded and to take more risks). Testosterone levels also respond to our social environment. Win a chess match, your testosterone goes up. Earn a million dollar bonus at your hedge fund, your testosterone goes up. Win the Master's tournament, your testosterone soars.