Susan has issues with guys in general at this point, but unfair as it may be to assume all dudes are awful, it's not entirely unreasonable that she feels that way because (SPOILERS!).
And if THAT doesn't convince you, here are some other reasons she feels that way at this point:
1 - She's had to deal with a lot of male jerks and they stand out more than the non-jerks. Seriously, if you put someone in a room with ten people and three of those people act loud and obnoxious while the rest are quiet, who among those ten people is that someone going to remember most clearly?
2 - She's best friends with Sarah and Sarah interacts with Tedd a lot and Tedd at this point in time hasn't exactly been doing a lot to make guys look good.
3 - She runs a feminist club. Now, before anyone leaps to their keyboards to tell me that feminist does not equal man-hating, I agree with you. Thing is, running such a club would involve reading and hearing about all sorts of issues women have to deal with throughout the world. It's not exactly the sort of information that would convince Susan she's mistaken in her views.
Commentary added October 8th, 2014