But, comparing drugs with sex, we find an obvious difference:
Sex is not dangerous, but healthy.
The danger for health connected with the drugs is the only rational reason to forbid it for children. This comparison makes sense only if we assume that some harm will be connected with sex if children are involved, even if the sexual relation is consensual.
This comparison may be used only for some special techniques or circumstances of sex which may be dangerous - like usafe sex or prostituation.