Why are people so homophobic?

today i was on the subway with my mother and there were to guys cuddling in the seats behind us. my homophobic mother started making stupid faces, and when i asked her what she was do in (i knew why she was...) and he said "i'll tell you later". she's a good person, but that seems to be her one tic. I don't get it, the guys weren't making out or anything, how could it bother someone?