Culture and individual behavior are a Catch-22
. Humans, by nature, are social animals. We unconsciously look for feedback from our own kind, and when that feedback leads to a tend, we tend to go with that trend. At some point, these trends may pick up momentum and be justified by social or legal stigma. For example, a straight kid that gets made fun of every time s/he does something that seems gay will eventually learn to stop doing those things and, possibly, will ridicule others that do the same.