If you're talking about this line :
If I profane with my unworthiest hand
This holy shrine,
Then the answer is that Romeo feels that he doesn't deserve to be in Juliet's presence . He uses the word "profane" to describe his hand , while he uses holy shrine to describe Juliet's hand . He is basically saying that he is unworthy that he doesn't deserve someone as pure as Juliet .
Hope it helps!