Homophobic Teens, Why Are You Homophobic?

Ive come across a lot of homophobic teenagers, I just wanted to know why. If it's because of your religion it is not valid, but it would make more sense if you don't support due to your religion.

EDIT: I'm sorry, I didn't mean to cause arguments. Homophobia is not valid, but I do understand why someone would think that way because of their religion. Also, I'm not downvote baiting or something, I'm genuinely curious.