If she has told you that she is not comfortable with you touching her like that, then you must respect that! That being said, as your relationship moves forward she may change her mind, but be prepared if she doesn't!
How do I get my girlfriend to let me touch her butt? We are 15, and have been dating for about 4 1/2 months. We have talked about it before, but not seriously, and she said she wouldn't be comfortable with my doing that until marriage.
If doing that is your sole purpose of having a relationship, then you don't deserve it. You need to learn the actual definition of what "love" is.
And like the other answerer stated, if she's not comfortable with you doing that, then don't do it. If you were uncomfortable with being roughly poked over and over again in the same spot, you wouldn't want that person to continue doing so, right?
What do you mean exactly, by 'Touching her butt' ?