Can someone explain to me this music video?

Okay at the end of Stacy's mom why does Stacy not look pissed that his mom is exposing herself to her daughter's boyfriend? Here's the link to stacy's mom ->a href="http://www.youtube.com/watch?v=dZLfasMPOU4" rel="nofollow"http://www.youtube.com/watch?v=dZLfasMPO…/a