You're mixing the reasons, sex, is merely for reproductive purposes, Greek cultures had an embedded paedophilia/homophilia as something totally natural, and only had sex with women to have children (or the same reason they would take a horse to make a long trip instead of walking... because THAT IS HOW IT IS DONE).

Now, the "cultural" sexuality is the part where some "rebel" is so rebel he likes to get stuff stuck into his digestive system backwards: SO rebel, that he does it in a place that is NOT designed to receive that kind of "input" leading to a nice income for the professionals fixing anal fissures.