Why do girls care if guys touch there butt?

The reason why is because it would make them feel uncomfortable and sometimes they may begin to think you might be just after one thing.

Also, a lot of girls are uncomfortable with their appearances, so maybe that's one of the parts of their body they don't exactly like.

Oh, and another thing, which is also another reason why a girl wouldn't some guy touching her butt is maybe because she has her period and you just made her extra uncomfortable.