They're brought up with constructed ideas of what masculinity and femininity are. Those ideas become ingrained in their identity. If something challenges that on any level they react negatively. Nearly all people are not 100% heterosexual or homosexual but instead exist on a spectrum. However if you don't want your constructed identity challenged by feelings you might be having that aren't heteronormative you're going to push those feelings away and anybody associated with them. Most of these homophobes probably have at least some non-heteronormative desires or fantasies and so work extra hard to fight against them so society doesn't perceive them as less masculine or less feminine. They would probably be so much happier if they just gave in and explored a bit.