Why does Romeo & Juliet begin as it does in A. I, S. 1, and why are women mentioned only as the butt of crude comments & boasting?
In Act 1 Scene 1, the arugment between Gregory and Sampson and the Capulet servents; followed by Benvolio's and Tybalt's entrances, the Prince's judgement and finally love-sick Romeo talking to Benvolio about his love.
You all ready have an answer, but I'll add this as a bonus anyway. Two for the price of one!
The introduction to Romeo & Juliet, Act I, Scene 1, is complex and accomplishes the introduction of all the essential elements that will follow during the course of the play in accord with the six Aristotelian parts of a play: plot, character, language, thought, spectacle, song. This accomplishment is constructed through the dialog and action of the servants, Benvolio, Tybalt, the Prince and Lords, Romeo.
The servants begin. They fill the function of the Shakespearean "Fool," who is savvy and urban and engages in intentional word play, as Sampson and Gregory do ("Clowns" are rural and engage in accidental word play). What these Fools accomplish in to establish the existence of a feud, its depth and breadth (even the servants) and its severity.
It is for this reason, to illustrate the feud's severity, that Sampson and Gregory talk not only about overpowering the male servants in fighting but also about overpowering female servants sexually (in addition, the ribald and crass always enters into Shakespeare's dialog because his audiences comprised all levels of society from beggar to King).
Next, Benvolio and Tybalt deepen the plot element introduced by the servants (feud) by establishing which house has what attitude toward the feud. The men of the house of Montague (Romeo's house), as represented by Benvolio, are the peacemakers (as is the Montague servant, Abraham: "Quarrel, sir! No, sir!"). The men of the house of Capulet (Juliet's house), as represented by Tybalt, are fightmongers (as are the Capulet servants, Sampson and Gregory: "My naked weapon is out: quarrel!")
This division of attitudes is ironically reversed in the two Lords, as Lord Capulet is the one who shows restraint, while Lord Montague is the aggressor: "He flourishes his blade in spite of me [my presence]!"
Romeo and Benvolio establish Rome Montague's view of the feud, which accords with Benvolio's peacemaker stance but is in opposition to his father's aggressor stance (Lord Montague). Romeo's stance is ironically in accord with Lord Capulet's stance of restraint (Juliet's father).
Further, Romeo's and Benvolio's conversation sets up the story line of love lost and love found, establishing that the tragic plot occurs in the midst of a love story. Their conversation also foreshadows what is to come: (1) finding a new love to replace his love who is sworn to chastity and so won't have a lover's romp with Romeo, which also foreshadows the theme of marriage and love; (2) that Romeo will not be taught to forget, which is integral to his final choices, the foreshadowing of which is confirmed by Benvolio's statement: "I'll pay that doctrine, or else die in debt," as he does die in debt because he has been unable to teach Romeo to forget.
The Prince demonstrates the high stakes in this love story tragedy: peace or death--regardless of circumstances. the love story is calculated to provide the worst possible circumstances for two houses who loathe each other to the point of murder, making the high stakes a very real and present threat. Additionally, the Prince tells upon what the quarrel is based, that being "an airy word from old Capulat and Montague," and how long the duration of the quarrel has been, "have thrice disturbed the streets." It is a new quarrel, even though Lord Montague calls it "ancient."
Okay there are a couple of perspectives from which we can look at this. In terms of the play itself, Act 1, Scene 1 is designed to convey the necessary exposition. With the servants running around in the streets with their weapons drawn fights will surely ensue, and this clearly sets up for the reader that there is strife between the houses of Montague and Capulet. Romeo and his remarks about Rosaline are used to beginning creating a sense of who Romeo is--a young man with a capricious nature. He proclaims his intense love for Rosaline and his disappointment that she does not reciprocate, and yet later that evening he is totally besotted by Juliet. His lack of maturity is a character flaw that we see throughout the play.
From the perspective of the true playwright and his audience, these scenes could serve a more practical function. They are written in such a way, that audience members who arrive late will be able to see what is going on, and the audience would more than likely be very quiet as they listened for the naughty jokes which is what would be of most interest to them perhaps.
As for the women being the butt of these bawdy jokes, it does not seem to be all women, but the lower class women such as Juliet's nurse.