Strong Immune System

There is a large body of evidence suggesting that sexual activity can affect the health of the immune system. That said, the reported effects of sex on the immune system are highly variable, differing across the sexes as well as across other factors, such as mental health.

Some of these differences may be due to the changes in immune function that occur across the menstrual cycle, which may have implications for fertility

That said, one of the few studies that looked explicitly at sexual frequency in young people found that more frequent sex increased the amount of IgA antibodies present in saliva—and possibly people's ability to fight infection.

It's important to note that one of the ways that sex could improve immune function is by reducing stress. High stress levels are well known to decrease immune function and increase the likeliness a person will become sick.

It's possible that the answer may be that sex improves immune function only when it's a stress reducer, but it isn't helpful when it's a source of stress,