There is, by now, a large literature on the chemistry and biology of love and sex. If you dive into that literature, you learn pretty quickly that our love lives are biased by all sorts of deep unconscious processes. When men become fathers, their testosterone levels drop, thus reducing their sex drive. There’s some evidence that it’s the smell of their own infants (but not other people’s infants) that sets this off.

Women, meanwhile, have different tastes at different times in their cycles. During ovulation, according to some research, they prefer ruggedly handsome and risky men, while at other times they are more drawn to pleasant-looking, nice men.