This definitely misses the power imbalance of punching down vs up. If someone genuinely believes all men are "scum," yeah, that's prejudiced. However, there is a big difference between a group that has less power in society pushing up against the class that has more power or oppresses them and the reverse. The idea that "y group is (insert pejorative)" and "x group is (insert pejorative)" are equally bigoted statements assumes that x and y groups are equal in social power. Statements like "men are trash" or equivalent don't necessarily represent an individual's true opinion of all men, but a general pushing back against a group with more power, many individuals of which attempt to exercise their perceived privilege over women.

Women that say "all men are trash" or similar might not be thinking with this level of introspection and subtlety, but it's a subconscious reaction to their position as a group with less power. They rarely hold that on a personal level against every individual man, unless they've been deeply hurt. I have experienced things that make it harder for me to trust men. My friends have experienced things that make it harder to trust men. I do not think every man is evil. When you see the damage around you on societal levels, see the people calling for your rights to be taken away, see how they treat you like an object or property because of who you are, and you see it in the lives of many many people like you, it creates a resentment of the group that is responsible.

I am not suggesting that there are no women that take advantage of men. I am not suggesting that men cannot be abused. I am not suggesting that it's okay to make men feel responsible for the actions of people that share only a gender with them, nothing else. However, I am explaining why women might feel hurt or disempowered enough to push back against men in general, and why "men are trash" and "women are trash" (though far more often, the phrase when targeted at women takes a sexual connotation: whores, etc) are not equivalent statements. Both the women that have been hurt and the men that feel hurt by the byproduct of their resentment are victims of the patriarchy. Until everyone, regardless of gender, holds the same societal power, there will always be people of all groups being hurt by the imbalance.

TLDR: Don't resent the women who are a product of their environment saying "men are trash," resent the patriarchy that hurts men and women alike.