resistance
In describing the psychology of sex, Rousseau begins with the premise that a successful sex act requires that the male feel strong. (The physiological basis for this claim is of course the fact that sexual intercourse requires that the male experience an erection.) And yet as Rousseau suggests below (para1257), the male's sense of his own sexual potency is sporadic; it needs the "psychological boost" (c.f. Joel Schwartz in The Sexual Politics of Jean-Jacques Rousseau) of overcoming woman's resistance. In the dialectics of intercourse, this logically means that the female must appear to resist so that the male feels as though he is overpowering her; he "triumphs from a victory that the other made him win."
The competitiveness and pride of amour-propre (which as we have been warned thoughout Book IV are inevitable with the appearance of the sexual drive at adolescence) brings an element of combat into the sex act; note the language of war in the descriptions that follow.