Generally, many men tend to judge two people before sex, and try their best to be attentive before sex. However, they only sleep after sex, just like a dead fish, almost dead tired. Few people can finish the work like the love masters in movies and television works, and still "this love continues".
In this regard, the female partners who are still not satisfied complain about the selfishness and lack of emotional appeal of men. However, from the perspective of biological evolution, the phenomenon that men fall asleep after orgasm may not be romantic, but just involuntary.
Secondly, some hormones in the male body will change after orgasm, which is mainly manifested by the increase of prolactin and oxytocin. These hormones are not only present in women, but also exist in normal men, just at different levels. Increased prolactin and oxytocin have the effect of promoting sleep. At the same time, γ- The change of aminobutyric acid secretion may also participate in the "hypnotic" effect.
Finally, because men's muscles are stronger than women's, they will "promote the combustion" of a large amount of glycogen converted into muscle energy during exercise, and the reduction of glycogen (possibly hypoglycemia) is also one of the reasons for promoting sleep. Of course, the male's control of breathing may lead to hypoxia during the course of sexual life, which may be the promoting factor of male's sleepiness after orgasm.
Although the above evidence is not absolutely sufficient, it can still show that the fatigue of men after orgasm is due to "being active and tired".