Benvolio is talking about Rosaline.
Explanation:
Romeo has not yet met Juliet at the Capulet party and he does not know about Juliet at this point in the play. The only woman that he loves is Rosaline but that changes when he sees Juliet.
An exclamatory sentence is a sentence you would be likely to "scream" in the real life: it expressed a great emotion, such as anger or surprise. An exclamatory sentence typically ends with an exclamation mark.
