There is no risk here.
When we say "skin to skin" contact, we mean mucous membrane to mucous membrane. Think oral, vaginal or anal sex, or unclothed rubbing/grinding, usually the kind associated with sex.
The skin on the upper butt is too thick for the virus to shed from, so if she didn't have an outbreak (assuming she has herpes), you wouldn't get it from touching her there with your penis.