I find it depends on the MMO and the people you're grouping with. Something like Call of Duty will get you a lot more dudebros then the Star Wars MMO but even there you've got the occasional arsehole.

I've been _very_ lucky as a gamer not to run into a lot of harassment but I also don't play a lot of FPS games. ME3 Multiplayers is probably the only one I regularly play. I'd like to play more but the misogynistic bullshit does put me off. :(