Some would argue that, yes, Romeo and Juliet were simply infatuated with one another and not that they were truly in love. There are a few reasons for this. First, the fact that Juliet, especially, is so young (she is not yet fourteen years old) leads some readers to believe that she cannot possible be really in love with Romeo because she doesn't have the emotional maturity for it.
Second, Romeo is nursing the wounds inflicted by Rosalind and the apparently unrequited love he feels for her. The fact that he literally seems to fall for Juliet on the same night that he is pining for Rosalind makes it seem like his feelings for either girl cannot possibly be love. Further, perhaps Romeo only grows attached to Juliet so quickly because she returns his interest, unlike Rosalind. He says something similar to Friar Lawrence, that his new love is better than his old because she feels likewise.
Third, the fact that the relationship between Romeo and Juliet progresses so quickly makes it seem too much like infatuation to some readers. They meet one night, get married the next morning, sleep together that night, and take their own lives so as not to have to live without the other a day or two later. It all goes so fast that it doesn't sound like love, which is thought to tend to build and develop somewhat more slowly.