Because people are homophobic idiots, that'ts why.
If you have religious motivations leading you to oppress a group of people, you're an idiot
If you're a teenager who uses "gay" as a derogatory term, you're an idiot
There's nothing wrong with being gay, problems only start when someone makes an issue of it, which is stupid. It's a loveual preference, and there's literally no reason to concern yourself with the loveual preferences of others as long as nobody is being harmed by said preferences.