Why does Juliet think that kissing for the second time is a sin?

This
isn't actually whatis saying. In her
playful conversation with , she teases him by saying that
although pilgrims
and saints do have lips, they should be using them for prayer, not kissing.

This is after Romeo has described his own lips as "pilgrims."

Juliet says primly that saints "grant for prayers' sake"that is, saints
stand
still and grant people's prayers. Whereupon Romeo says, rather
cheekily, that she should stand
still while he takes what he's prayed forthat
is, a kiss. In doing so, he says, "my sin is
purged."

This
is where the sin comes inJuliet complains that, while
Romeo's sin may now be
gone, Juliet's lips have "the sin that they have took."
Essentially, she's
saying "maybe your sin's gone, but look, now it's on me instead!"
Romeo then
takes this as the invitation it is, saying that in that case, he'll have his
sin
back"give me my sin again." It's an excuse for him to give Juliet another
kiss, under
the guise of taking his own sin back from her. Throughout the
conversation, they're discussing
"sin" as if it's a piece of chewing gum
being swapped between them, rather than with
any
seriousness.