Primates have large brains. Reproduction is rare because pregnancy is hard. Because of this, sex is not confined to reproduction, although unitive sexuality is important ultimately to it to maintain relationships between pregnancies. Monogamy does not spread disease. As for the ancients, Leviticus was written to differentiate Jews from Babylonians during the exile, lest assimiliation leave no one to return to Israel. Even then, sexual teaching-was political.