Because it is against the law... its called statutory rape. She does not have ability to consent to sex with an adult. minors need to be protected from manipulation from adults who just want to get their jollies. and BTW Michelle... where do you draw the line in 2012? Is it okay for this 18 yr old to say engage in a sexual relationship with a 12 yr old, 10 yr old, 5 yr old? In this country we have set the age of consent to be in the range of 16 to 18 for these very reasons.