As it turns out, men send signals. One of these signals is smell. The immune system sends out smells, carried in sweat, that women seem very attracted to. In one study on sweat, men wore the same T-shirt for a number of days and women rated the smells in terms of sexiness. Very consistently, the women picked men with immune systems most different from their own.
Some people think the same immune-system information is found in saliva.
So, French kissing may be another way of discerning this biology. However, the difference in aggression is there right from the beginning. Even prenatally, male fetuses are more active. Early on, we were a polygamous species. Certain studies show that male animals, including humans, evince a noticeable drop in testosterone after being paired off in committed sexual relationships.