So, you're the hero, or, more frequently, the heroine, and you fall madly in love. The only problem is : he (or she) is married or in couple. Your moral code should prevent you from going out with him or her. Except, of course, for a little thing : if the spouse is the Alpha Bitch, The Baroness, or a Manipulative Bitch (or the Jerk Jock, or a Villain, or the Manipulative Bastard), you are allowed by your audience to disregard her/his feelings, their ten years-long and successful marriage, and the mental balance of their three children. Just throw yourself on him (or her) !

Subtrope of Protagonist Centered Morality and Sympathetic Adulterer.