Apart from concerted sociopolitical efforts to make an underclass that can be harmed at will and blamed for everything, there’s a couple of less deliberately malicious explanation for a lot of people.

1: Category violation -> perversion -> predator -> monster.

2: I don’t think it’s real -> they are lying -> why would they lie about gender -> to lull people into a false sense of security -> predator -> monster

Both are of course incredibly false and hateful, and both of course are pushed maliciously and cynically by many, but I think some people are actually taken in by one or other of these.