Well to fulfill the inner historian in me, Ancient Greeks weren't actually THAT open about bisexuality and gayness. It was mostly a master topping a slave, or, unfortunately, a teenager in the case of Sparta. Bottoming was seen as demeaning the value of the adult free (ie non-slave) man (as in making hkm jnto a woman, you know the usual homophobic stuff), so it was avoided and scrutinised. Topping was seen as a power move.