The 'wherefore' here means why rather than where. What Juliet is asking, in allusion to the feud between her Capulet family and Romeo's Montague clan, is 'Romeo, why are you a Montague?'. Their love is impossible because of their family names and she asks him to change his allegiance, or else she will change hers.
http://www.phrases.org.uk/meanings/262200.html
Magnificent Montague:
Burn, baby, burn. A slogan adopted by militant Blacks after a riot in Los Angeles in 1965, in which 34 people died and wide area was destroyed by arsonists.
From _Brewer's Dictionary of 20th-Century Phrase & Fable_ (1991)