Libby's mom lives with her boyfriend, and sex is implied, but nothing ever seen.

Libby herself is seen quite a few times where it is obvious she is braless. It is also strongly implied that she fornicates with one or two different boys. This is while Libby is a minor, a high-school aged girl, and the boys are above 21. The actress was actually 20-21 at the time. Nothing seen at all.