This can really depend on what your current relationship is with the girl. If she is your girlfriend or you are dating then of course you have a better chance of being allowed to touch her butt. However, if you are only a friend or a relative stranger to her it will be a lot harder, if not impossible, to touch her bottom without her being rather annoyed or disgusted in you.
Firstly, if she is your girlfriend, then although she has the right not to let you touch her, it should be okay to touch her butt if you either ask or do it slowly and hint you are about to do it. Women can feel violated if you just go ahead and grope without any prior warning or asking permission. This could get you a slap in the face or even end your relationship if she feels you don't respect her and just see her as your property. It can also be construed as sexual harassment.
The best policy is to always be respectful and ask her for permission. You may find she will be flattered that you find her body so attractive that you want to touch it. However, if it is a stranger, I would not advise trying this as it could get you in a lot of trouble and could result in a criminal charge of indecent assault.
Firstly, if she is your girlfriend, then although she has the right not to let you touch her, it should be okay to touch her butt if you either ask or do it slowly and hint you are about to do it. Women can feel violated if you just go ahead and grope without any prior warning or asking permission. This could get you a slap in the face or even end your relationship if she feels you don't respect her and just see her as your property. It can also be construed as sexual harassment.
The best policy is to always be respectful and ask her for permission. You may find she will be flattered that you find her body so attractive that you want to touch it. However, if it is a stranger, I would not advise trying this as it could get you in a lot of trouble and could result in a criminal charge of indecent assault.