Most Helpful Girl
Both are equally bad, but I guess it's more common for men to cheat so they're always the ones getting bashed for being cheaters. In the long run, it doesn't feel good to be cheated it on. Doesn't matter whether it was a boyfriend or girlfriend.
Most Helpful Guy
Culturally women. The only reason I say that is due to Maosource's comment specifically being a mentality that's widespread. In my society if a woman yells at, hits, cheats on, or otherwise undermines a man it is commonly accepted that he "deserved it" somehow.
The flipside is also ignored where if a woman fails to meet the emotional needs of her partner or have a sensible presence within the relationship on all fronts if the male should cheat he is a dog looking not for emotional comfort and wholeness but a piece of ass.
I was just going to say this anyway but then someone actually showed up and proved how integrated into society this thought can be. It is politically incorrect to say there is a difference but most people will side with the woman and her (even if dirt poor) reasoning.