Several years ago I use to receive a medical journal called Medical Aspects of Human Sexuality. I don't remember all of their criteria for cheating, but here are some.
1. Don't screw a neighbor or relative by marriage. (inlaws)
2. Don't screw around in your town.
3. Don't go with someone that abuses alcohol or drugs.
4. Don't go with someone that has mental issues.
5. If you are married make sure your lover is also married.