After they leave the Capulets' party, Romeo abandons his friends to climb the wall into Juliet's garden and, he hopes, to see her again. Benvolio and Mercutio call to him, though Benvolio is more concerned about him while Mercutio just teases him. They do not know that he has fallen in love with Juliet and they believe that he is still lusting after Rosaline. Mercutio tells Benvolio,
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. (2.1.33-36)
Literally, he means that if love is blind--as Benvolio has said--then it cannot hit its target. For him, and because he believes Romeo is still pining for Rosaline, who has declared that she will not have sex with Romeo, Mercutio really means "lust" when he says "love," and the target of lust is sex. Mercutio doesn't realize that Romeo loves Juliet and doesn't just lust after her, and so Mercutio describes what he believes to be Romeo's desire for a mistress who would be free with her sexual favors.