Your comments about serotonin levels etc. don’t actually explain why, merely how levels of sexual desire increase in the summer months. The real reason we humans become more horny around May and June is because the human gestational period is 9 months. For most of our evolutionary history we didn’t have the luxury of houses and central heating and supermarkets and doctors. We lived precarious lives, subject to the elements. Think about the chances of survival for a child born in November (in the northern hemisphere): food is scarce, it’s cold. Not a great set of odds. Conversely think about a child born in March or April: a good six to seven months of increasingly warm weather and increasingly bountiful food supplies. Those first six months of life, in a resource-rich environment, often made the difference between life and death. So parents whose genes pushed them to mate in the summer had more offspring than those whose genes pushed them to mate at other times of year. Over the millennia the former out-bred the latter, leading to the “fuck when it’s hot” genes to predominate in the gene pool.
This is why we humans, Caucasian ones at least, have evolved to feel more horny in late Spring and in Summer.