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Mercutio is Romeo's sword-fight loving BFF, and you probably won't be surprised to find out that his name sounds a lot like the word "mercurial," i.e. "volatile," i.e. "touchy." He never backs down from a duel and, although he's neither a Montague nor a Capulet, he gets involved in the long-standing family feud on the side of the Montagues. That turns out to be a mistake on his part: Tybalt kills him in Act 3, Scene 1.
Mercutio is a showstopper. He's dirty, funny, out of control, and—we'll say it—compared to him, Romeo and Juliet can seem whiny and repetitive. Mercutio is technically a minor character, but his personality has such a disproportionate impact that maybe he has to die or he would take over the play. In fact, English poet John Dryden said that Shakespeare himself admitted that he had to kill Mercutio—or else, he said, Mercutio would have killed him.
True? Probably not. But it helps us get an idea of just how flashy Mercutio is. Just take his entry into the play. He's needling Romeo about not wanting to dance, and basically the first thing he says is "Prick love for pricking, and you beat love down" (1.4.28). In other words—the best way to cure a broken heart is to have sex.
It's not surprising that, with this attitude toward love and sex, Mercutio comes across as opposed to the whole idea of love between a man and a woman. When Romeo complains about the heartache of his unrequited love for Rosaline, Mercutio tells him to get over it already:
If love be rough with you, be rough with love.
Prick love for pricking, and you beat love down. (1.4.27-28)
It's not just "love" that Mercutio has a problem with. He's also pretty hostile toward women and female sexuality in general. The clearest example of this is when he lists Rosaline's body parts in a crude monologue that makes fun of Romeo and a popular poetic convention (the "blazon," a poetic technique that catalogues a woman's body parts and compares them to things in nature):
I must conjure him.
I conjure thee by Rosaline's bright eyes,
By her high forehead and her scarlet lip,
By her fine foot, straight leg, and quivering thigh,
And the demesnes that there adjacent lie,
That in thy likeness thou appear to us! (2.1.19-24)
So, is Mercutio's hostility toward women and heterosexual love an indication that he's more interested in the guys? Maybe. He does make a lot of jokes about penises, and Tybalt does claim that Mercutio "consortest with Romeo" (3.1.46). On the other hand, maybe it doesn't make much sense to apply 21st century categories of sexual desire to a 16th century play. One thing is certain: Shakespeare's work is full of men who value male friendship and comradery over male-female relationships.
Other interpretations put a psychological spin on Mercutio's strange, imaginative rants. Take his Queen Mab speech, which starts off as a bizarre rant about the mythological fairy Queen Mab and ends up talking about (of course) sex:
This is that very Mab
that plats the manes of horses in the night
And bakes the elflocks in foul sluttish hairs,
Which once untangled, much misfortune bodes.
This is the had, when maids lie on their backs,
That presses them and learns them first to bear,
Making them women of good carriage. (1.4.93-99)
The backstory here is that episodes of sleep paralysis were often explained as a demon or succubus sitting on the sufferer's chest—and possibly having sex with him or her. Here, Mercutio says that women who are "hag-ridden" by Queen Mab are just learning how to "bear"—i.e., bear the weight of a man on top of them and, by extension, bear children. The take-home point, we think, is that for Mercutio, sex is always a little gross and dirty—and always a little crazy.