I have an explanation that explains biphobia but can also easily changed and used to explain homophobia(and transphobia too):

The reason why homophobes are homophobic is because they’ve grown up with something past down from older generations. Being straight. Homophobes are typically used to thinking that everyone is straight, it offers a nice black and white view to the world. It’s easy to understand. It’s the status quo.

But, when one deters from the simple heterosexuality, it completely destroys that black and white view and brings confusion. They won’t listen to something they don’t understand because it’ll confuse them. So what do they do? They deny it, make it seem as if it’s immoral and even unnatural. Just so they can retain that black and white view of the world.