They’re two college students engaged in a consensual and (AFAWK) monogamous sexual relationship. And judging from the fact that (again, AFAWK) Stef hasn’t gotten pregnant, they’re being safe when they do it. Somebody report them to the ethics police!
A memo to parents everywhere: If you’re going to be that kind of mom or dad, don’t expect your kids to talk to you about anything going on in their lives.
In the third panel, she’s imagining going much farther than a hickey.