“Sexual intercourse can sometimes lead to a feeling of melancholy called PCT, or post-coital tristesse (from Latin post-coital, and French tristesse, literally — “sadness”). This is more common in men than in women. Many PCT sufferers may also exhibit strong feelings of anxiety, anywhere from five minutes, to two hours after coitus.
The phenomenon is referred to by the philosopher Baruch Spinoza in his Tractatus de Intellectus Emendatione when he said “For as far as sensual pleasure is concerned, the mind is so caught up in it, as if at peace in a [true] good, that it is quite prevented from thinking of anything else. But after the enjoyment of sensual pleasure is past, the greatest sadness follows. If this does not completely engross, still it thoroughly confuses and dulls the mind.”
A better-known reference is the Latin phrase Post coitum omne animal triste est —”After sexual intercourse every animal is sad”.