I've discussed this with others & even written about it. To start with I agree that misogyny & misandry are not equal & that men don't face the same challenges & dangers—your stag/hen party analogy was spot on. Man-hating might be a form of prejudice, but it's not sexism.
Still, I have to disagree with the suggestion that hating men (or hating anyone, actually) is a reaction to the their behavior, rather than a conscious decision on someone's part.
Implying that any woman who hates men must have been mistreated by them reduces their complexity. I.e. there are doubtless many women who have never been mistreated by men who find us gross, alien & simply unpleasant for reasons of their own. Why not? They're entitled.
Furthermore, I know many women who have been mistreated, badly mistreated, by men who don't hate us.
When we argue inevitable bias based on mistreatment, where does it end? Over on American Renaissance, a white supremacist website, readers cherish accounts by "one-time liberals" who had their eyes opened by being victims of crime or other misbehavior at the hands of people of color. Or in some instances, just from reading the news & focusing on accounts of crime involving people of color. As a result, they have now become "race realists".
While I never doubt the essential truth of their accounts, I suspect confirmation bias plays into their choice to embrace white supremacy. I believe it does in most choices to embrace a prejudice.