In addition to copulins, let's not forget pheromones and their role in influencing behavior.
Sex seems to be a short cut to the reptilian and paleomammalian complexes.
While we are at it, you might as well throw in:
1) Hypergamy - Alpha lays, Beta pays
2) Briffault's Law
3) Solipsism
4) Eggs are expensive. Sperm is cheap.
I suspect the ancients were keenly ware of the above hence the structures that were put in place to allow civilization to develop.