My sneaking suspicion is that human beings, like our amorous bonobo cousins, had, at one time, expressed ourselves much more sexually with one another. I don't think our longing for intimate contact with multiple partners has changed. We have, however, learned to feel a certain measure of guilt and apprehension about it. My theory is that today's strict sexual controls evolved alongside the organisms that began to take advantage of early man's promiscuity. Rather than instruct people to be more discerning, religious and cultural leaders instead took the hard line (maybe not initially, but eventually). Those who wished to remain within the good graces of their neighbors complied with the new regulations and shunned those who refused. That which was once good in the interest of public safety became bad.