When a man has a long, tender and passionate relationship with a woman he loves deeply, but still refuses to leave his wife because he has made a solemn commitment to her that must be honored, even at the price of his own happiness, and the happiness of his beloved.
I'm not trying to be facetious, I'm just trying to make a point. This is not the kind of question that has only one answer.
Or, being facetious:
Q: If your wife and your mistress were both drowning, and you could only save one of them, who would it be?
A: My wife, obviously.
Q: And why would you save her, and not your mistress?
A: Because my mistress would understand.