Benvolio and Mercutio are making fun of Rosaline to make Romeo come back to them. They figured if they spoke badly about his former "love", he would get offended and come back to tell them to stop.
Rosaline is the ugliest woman ever
I know!!! Could she get any worse!?!?
ACT II SCENE II
Romeo is in the Capulets's garden watching Juliet confess her feelings for him on her balcony. Juliet finds Romeo and he wants to marry her. She has second thoughts, but then decides to marry him in the morning.
I love you!!!!!
Then marry me!!!!
Right after Juliet wants to marry Romeo, he goes to Friar Laurence and asks if he can wed him and Juliet. He agrees, but is surprised that Romeo is over Rosaline so fast.