Quote 4
MERCUTIO
This [Queen Mab] is the hag, when maids lie on their backs,
That presses them and learns them first to bear,
Making them women of good carriage.
This is she—
(1.4.97-100)
Mercutio doesn't see much to laugh about. To him, sex is almost literally madness—and an oppressive one, like Queen Mab—the love-fairy—weighing down virgins while they sleep.
Quote 5
MERCUTIO
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.
(2.1.20-24)
We managed to read this without blushing, but Mercutio is actually being pretty insulting here: he's breaking Rosaline down into parts like the popular poetic blazon http://www.ic.arizona.edu/ic/mcbride/ws200/renlyric.htm, but he's being dirty about it. The "demesnes" that like "adjacent" to her "quivering thigh" are her genitals. (Makes you wonder if the people assigning Romeo and Juliet in high school actually understand Shakespeare, doesn't it?)
Quote 6
MERCUTIO
If love be blind, love cannot hit the mark.
Now will he sit under a medlar tree
And wish his mistress were that kind of fruit
As maids call medlars, when they laugh alone.—
O Romeo, that she were, O, that she were
An open-arse, thou a pop'rin pear.
(2.1.36-41)
What, you don't get why this is funny? Let us explain: A "medlar" is a fruit that looks—to the Elizabethans, at least, like a certain body part—so much so, that they called it an "open-arse" (which would almost certainly have meant female genitalia, and not what we'd associated with "arse.") And then there's the "open et caetera," which means, well, an open vagina; and a "poperin pear," which sounds suspiciously like "pop-her-in." In other words, Mercutio wishes Romeo's mistress were sexually available to him.