Romeo fell in love with Juliet before he found out she was a Capulet. It actually would have been easier for him if she had not been of equal status, or at least not been a Capulet, because it is harder to marry a woman of status. Her parents were interested in having her marry someone of equal status.
Romeo is completely enamored of Juliet before he knows who she is, and she returns his affection but does not tell him her name. When Nurse says her mother wants her, Romeo asks a servant who her mother is.
Is she a Capulet?(125)
O dear account! my life is my foe's debt. (enotes etext pdf p. 33)
Romeo is upset because she is of the feuding family. He does not care which family she is from. Romeo seems to fall in love easily and fully, and although he clearly cares about beauty he may not care as much about class. It would have been clear that Juliet was a member of the upper class by how she was dressed and the fact that she was invited to the party, and he might have assumed that was enough.