I doubt that's actually a reason many people hold. It's more of an excuse. The basic reason is probably common to most anti-gays - simply that gay sex is disgusting and weird and they can't bare to imagine it. If it wasn't that, they'd be supporting gay marriage but trying to restrict effeminate males, "butch" women and single parents from raising children. The "it's disgusting and weird" reason also explains the extreme prejudice against minority sexualities even from members of the pro-gay community.