In a study of 223 women and 76 men, participants were given a list of 21 possible postcoital symptoms. Among women, some of the most common were mood swings and sadness; among men, feelings of unhappiness and low energy were common. Participants reported other experiences as well, leading the authors to suggest the more neutral term postcoital symptoms rather than reducing everything to dysphoria.
Importantly, 73.5% of participants who reported these symptoms said they had occurred at least sometimes after consensual sexual activity. At the same time, the study used a small, non-representative convenience sample, so those percentages cannot be generalised to the whole population.
Science still does not have one explanation for why this happens. That is why popular stories about a sudden drop in a particular hormone after sex should be treated cautiously when they are presented as an established cause of PCD.
What matters more to me is the basic fact: emotional experience after sex can be more complicated than a rating of “good / bad.”
Someone can experience pleasure and then sadness. The human nervous system is perfectly capable of holding both in the same evening.