have an affair with someone 与某人有暧昧关系,有外遇
She used to have an affair with one of her husband's friends.
cheat on someone 不忠实于 (配偶或伴侣)
He’s cheating on his wife.
step out on someone 对伴侣不忠
Hank has been stepping out on Sophia, and she doesn’t know it yet.