groom doesn't know how old his fiance's sister is?

after colin hanks bangs the fiance's little sister, she complains to the groom that his friend banged her little sister. He says great, whats the problem. She says she is only 17 and it sounded like he was learning this for the first time. How does a guy in a relationship for almost 5 years with a girl not know how old her little sister is?