As math requires, men and women cheat in equal numbers:

[Researchers] gave 203 young heterosexual couples confidential questionnaires asking them whether they had ever strayed, and whether they suspected or knew their partner had strayed. In this, 29 per cent of men said they had cheated, compared with 18.5 per cent of women.  The men were better than women at judging fidelity. "Eighty per cent of women’s inferences about fidelity or infidelity were correct, but men were even better, accurate 94 per cent of the time" … However, men were also more likely to suspect infidelity when there was none. … Complex statistical analysis of the data hinted that a further 10 per cent of the women in the study had cheated on top of the 18.5 per cent who admitted to it in the questionnaires, whereas the men had been honest about their philandering.

So why are men more honest than women in cheating surveys?