How is male aggression presented in Romeo and Juliet in the conversation between Sampson and Gregory?

Samson and Gregory's macho talk is certainly aggressive and disturbing, a foretaste of the masculine aggression to come that will cause trouble for the main characters in the play. They are servants of the Capulet house and talk about killing and raping the female Montague servants as though it were all a matter of jest. Then they provoke a fight between themselves and some Montague servants just to spite their enemies.
The aggressiveness in this conversation sets up the feud for the first time after the prologue, putting into concrete, ugly terms the enmity between the Montagues and the Capulets and showing how this hatred affects even the servants of the households. It establishes a sense of danger right away too: these people are willing to humiliate, hurt, and slaughter one another just on a whim. There is nothing honorable or justified about any of the aggression here.
This aggression also infects the gentle Romeo in both small and large ways. He makes crude sexual remarks with his friends, which are a great contrast to his sweet words for Juliet, and he later kills Tybalt in vengeful anger even after the joy of marrying Juliet, who it would seem had almost tamed him of such violence.

In act 1, scene 1 Sampson, a Capulet, and his friend Gregory show male aggression in a conversation about their hatred of the Montagues. Sampson refers to a Montague as a "dog," says the Montagues make him angry, and threatens to walk against the walls of the streets to force the Montagues into the gutter.
When Gregory responds that only "the weakest goes to the wall," Sampson agrees. He responds by saying he will push the Montague women up against the wall. As the swaggering, aggressive talk escalates, Sampson says he will cut off the women's ("maid's") heads. Gregory asks if he really means he will cut off the women's heads, and Sampson responds with a pun:

Ay, the heads of the maids, or their maidenheads.

By this he means he will either behead the women or rape them. He also states he will provoke the men:

I will bite my thumb at them; which is a disgrace to them, if they bear it.

When Montagues do appear, Sampson is worked up enough to provoke them, and a street brawl breaks out.
It is notable that as the aggressive talk escalates, it encompasses violence against women as well as men. Sampson's desire to rape the Montague women is a contrast to Romeo's love for Juliet.