She fell in love with him before she knew who he, in fact she has every reason to believe that he is not a Montague. "Forbidden Love" is not a theme of the play, although the fact them come from feuding families makes their relationship difficult, it is not a driving theme. When she does find out who he is, she goes through the 'rose by any other name' speech not because he is 'forbidden' but because she knows that it makes it difficult.
https://answers.yahoo.com/question/index?qid=20110407063353AA2eVlm
