Because they’re uncomfortable, and anything which makes them feel uncomfortable is perceived as a moral failure on the part of the person making them uncomfortable, and it’s usually a short jump from that to violent threat and/or moral corruption.
But they can’t give a real reason. Or, at least not one which makes sense to, or is likely to be seen as valid by, members of the general public. Danger to society? People are wary of that type of argument because it’s usually proven false in the past, and often the subject is one where even political centrists have long moved on. Because God says? Religious fundies are finding their grasp on society becoming ever more tenuous. Danger to women? Tradcons and TERFs are both finding that, in an age of slowly creeping equality, that argument is holding less and less weight, because it relies on the presumption that women are helpless. Danger to children? Disgust at bestiality? Occasionally something outré like cannibalism, necrophilia, or coprophilia? Those still hold some weight.
Their attempts to tie their discomfort to any of those things doesn’t always work nearly as well as they hope, though. Regardless, they usually need to convince themselves before they can convince others.