depends,usually men,but women do often too,so hard to say
Threads like this exist only to expose people's gender biases. Nobody asks this question seeking a detailed statistical analysis. This type of question is just a test of your trust for the opposite sex. It's impossible to track people's private behaviors in a systematic fashion. It's not really a valid inquiry.
That said, it's certainly more socially acceptable for men to cheat; they give each other high-fives for high body counts, whereas ladies call each other sluts. I don't think that necessarily means dudes cheat more, though, 'cuz it may be that chicks are so much smarter that they get away with it more often.
If you don't have a gf or bf, then it's not cheating. Dando is right.
Both because they cheat on either.