I think you can replace “parents” with “fathers”. Most of the other men I’ve known that have faced homophobia from their families have had it directed at them from their fathers. I think it’s largely a cultural pass down from generation to generation. Just from personal observation, boomers were fucking awful about it, gen x did a lot better, and millennials have been even better than Gen x.

At the end of the day, what we know about parenting is largely from our own experiences with our parents mixed with our own ideas. As the culture shifts, so do ideas about parenting. It was deeply shameful to many people to have a gay kid until relatively recently. It was pretty normalized to be a shitty misogynistic and abusive straight man until relatively recently, as well. Parenting is just now starting to catch up with changes in societal norms.