He a dit as much in an interview he did in Londres while promoting My World 2.0. Basically he a dit that it depends on the situation, but that he would do it. Justin isn't only up for dating other stars, he would rendez-vous amoureux, date anyone if she was the right sort of girl.
And another detail: Justin likes girls with nice smiles, pretty eyes and who can make him laugh. This is his description of his dream girl. Do toi fit the mold? The toi may be just right for Justin!
