The National Couples Survey asked 234 whites if they have had sex with someone other than their spouse. Here are the estimates by how religious the respondent is:
Not religious 14.3
Somewhat religious 3.3
Very religious 5.4
Those who are not religious are 4.3 times more likely to have strayed than the moderately religious and 2.6 times more likely than the very religious. (Differences are not statistically significant.)
The finding is consistent with my General Social Survey analyses showing a positive link between never going to church and infidelity, and atheism and cheating.
(These numbers might look low to you. Keep in mind that the age range for females is 20-35 and the males (their partners) are ages 18 and up, so this is a young group with fewer years of marriage than the general married population.)