Well, an assumed connection between legality and morality is perhaps part of the disconnect.

Having sex with 16 and 17 year olds is not morally wrong because they are legally considered minors and you an adult.

Having sex with 16 and 17 year olds is considered morally wrong, by some, because the state of their mental maturity is often far below that of an adult even 5 years older. Consider the biological “growth” of the human brain; having not fully developed, their behavioral maturity follows suit. This means their ability to reason, their ability to act in their own best interest, is less than your own. At a certain level of disparity, this imbalance is usually considered “taking advantage” of someone. Asking them to do things they have less an ability to understand than yourself starts to look a lot like coercion.