I believe some people are born attracted to the same sex or become that way early on. We can often tell if someone is gay without having witnessed them doing or saying anything. The existence of "GAYDAR" and its high level of accuracy mean that gay people don't just behave differently, they are different.
Now the kicker. There have been studies done on sets of identical twins and there are many cases where one twin is gay and the other isn't. They have the exact same genetic makeup so it's not a "gay gene". That leaves something happening in the birth process or early life as a real possibility.
Sparta was as much into pederasty as Athens was if not more so. That is not a good comparison.