I would say it has to do with respect. Groping is never respectful in public (other persons have to witness and they may not wish to; the same goes for public nudity) and usually not in private unless there's clear communication that it would be welcomed.

Killing other people is not very respectful of them and, a great many of us are here because we had sexual experiences "too young" so that's a more scientific and practical matter, when is a human mature enough to deal with the emotional and other consequences of sex. It's nice that teens can take their babies to school now but, generally it's not so respectful of those paying taxes to literally have to pay for the teen's deliberate choice to disrespect.