Not always - in some states, with parental consent, they get married. Is it the age difference, or the outside-of-marriage, promiscuous sex that's the problem? Perhaps our society needs to rethink its objections to marriage of 16-19 year olds.
That is just the point. You and I are arguing about the "rules". The framework and rules don't exist in the homosexual community. Don't confuse cultural values with laws. We are far more influenced by cultural "rules" then by laws.