For it to be cheating you need to be married (or in an exclusive relationship) in the first place. If the other party is not, then they're not cheating anyone themselves.
So, if it mostly happens between married men and unmarried women, for example, cheaters don't have to be "roughly equal".
As for cheating with married women, in China woman's infidelity is much lower than that of married men infidelity.
"In a representative sample of urban Chinese – 3.9% of married women and 20.6% of married men reported to have the experience of extramarital sex in the past year".
So just because each dick needs a corresponding vagina (and vice versa) doesn't mean that infidelity between sexes is equal. There are other factors in play too.