Many people have tried to express the difference between

1)things that are made crimes because they infringe directly on other people in some way - such as theft, murder and fraud,

2) things that are crimes because they directly neglect causal factors that should be known to predictably lead to harm to others, such as criminal negligence for causing a road accident while polishing your nails at the wheel, and

3) things that are crimes because the powers that be think that though they may or may not predictably cause direct harm or immediate negligent harm to others, they turn people enough away from social concerns and into their personal pleasures that the net effect is to dissolve society, and possibly also to allow a neglectful attitude to develop that leads to diverse crimes and vices secondarily. This is what I referred to as "crimes of social weakness."

Whereas a case can easily be made that sex with young children is in category 1, responding favourably to an invitation from a 16 year old who finds you attractive is clearly in category 3. You are merely thought to be bad for the person's moral formation; his (relative) "innocence," which is the state of pre-military lack of addiction to the major sensual pleasures of adulthood, is the item you arguably assault by granting him his wish.

In effect, from this 'Gibbons, Fall of the Roman Empire' viewpoint, you un-soldier him, you allow him to fall into pleasure-seeking. Of course, by 21 everyone except the more honest Catholic priests has done the same fall or tried very hard to, so it's mainly a matter of unmanning a person (in Victorian mythology) at the height of their ephebic development. Oh my gosh, now they will NEVER learn self-control -- their sexual bed will be unmade forever, boots unpolished, commands from the sergeant only taken with attitude, not true compliance.

If you subscribe to this mythology about how social cohesion forms, then this IS a crime. If you think people can become good citizens even though they've had sex with another male at age 16, then the whole thing is pitiable balderdash.