There is no such thing as a casual, non-significant sexual act; everyone knows this. Contrast sex with eating - you're strolling along a lane, you see a mushroom on a bank as you pass by, you know about mushrooms, you pick it and you eat it quite casually - sex is never like that. That's why virtue in connection with eating is basically a matter only of the pattern of one's eating habits. But virtue in sex - chastity - is not only a matter of such a pattern, that is of its role in a pair of lives. A single sexual action can be bad even without regard to its context, its further intentions and its motives.

Contraception and Chastity (1975)