Adultery
Adultery is when you are married and have sex (or sexual activity of any kind) with a person who's not your husband or wife, or when you have sex with someone who's married and not to you. If you're not married but have a committed, exclusive sex partner this is called cheating. (And sexual exclusivity is to be assumed in a sexual relationship?.) Adultery and cheating are also called infidelity.
People have argued what kind of sexual activity constitutes adultery, but I argue if it's sexual? at all it's adultery, though maybe just a little, even sexual looking.
God/life/the universe/everything seems to punish the major adultery with 7 years of suffering, though I think not for looking. I don't recommend it.
"I did not have sexual relations with that woman." — Former US president Bill Clinton, but he did