If we could travel back to when critters diverged into interior sperm depositation, therein lies the first of the fittest. resources were conserved, a next generation virtually guaranteed. Sex is wired into the amygdala of our brain, the oldest section of the brain and the first to appear in the head of animals: Most likely lizards, the oldest living example of evolutiion.
They as we smell our objects of sexual desire, thru a veromosnal nerve which comes from the nose,the only nerve wired directly to the brain. We actually smell a receptive person, and if the chemistry is right, they will mate, in a frenzy of sexual congresses. When the female becomes inpregnated, her smell changes, the male is no longer turned on,
except for the fact we are actually meant to be mostly monogamous, and the feel of her vagina around his penis, makes him fall in love and stay.
Does not destroy that pesky veromonasal nerve going straight back to his brain. All of a sudden he has a choice, head for the next available, ready female, or stay with the one he is with, usually he will try to do both.