I would say that Juliet's immediate reaction is best described as coy (a mixture of shy and inviting at the same time). When Romeo first sees her, he takes her hand and then asks to kiss her. her first response, "Good pilgrim, you do wrong your hand too much," shows that she is surprised by this gesture. She is politely telling him that taking her hand and asking for a kiss is an act of devotion, that he does not need to apologize for, even though they have just met.
As they continue this playful banter, he again suggests a kiss:
"O, then, dear saint, let lips do what hands do.
They pray; grant thou, lest faith turn to despair."
Juliet is too well-mannered to tell Romeo he can kiss her, but she does not discourage him from doing so. After their kiss, Juliet tells him, "You kiss by th' book," which means he is a good kisser. This line is a good example of how Juliet is definitely inviting Romeo's continued advances.
Soon after the two are seperated and Juliet learns that he is a Montague. Her famous aside shows her immediate reaction to this news:
"My only love sprung from my only hate!
Too early seen unknown, and known too late!
Prodigious birth of love it is to me,
That I must love a loathèd enemy."
This is the line that demonstrates love at first sight. Though they have only spoken a few lines to each other and exchanged two kisses, she is already hooked and doomed, which is clear from the line, "known too late!" He is the only boy she has ever had such strong feelings for, but he is her family's "loathed" enemy.