It's an outward expression of Conservative hierarchical values (I rant about this too much maybe..). Conservatives conserve the traditional hierarchies of society, for example God over man, men over women, straight over lgb, white over PoC, self over society, rich over poor etc. For them, people listening to Greta represents a perversion of the traditional hierarchy. Why would they listen to a young woman, advising the betterment of society? She's not an older man, she's not saying get rich and think of yourself, she occupies a low spot in multiple hierarchies so she must be wrong. But it isn't enough for her to be wrong, she needs to be dominated because that's how hierarchies work, the higher levels dominate the lower. Sexual violence is one of the classic forms of human dominance, so they don't imagine beating her in an argument, they imagine raping her into submission instead.

They think that somehow if they rape her it will magically make them better than her and that she will submit and agree she was wrong. It's a mentality only the truly weak have.