I argue that such boundaries are pure convention.
The main argument are the great cultural differences. The other great differences even between people from a single culture, dependent on their different sexual education.
Sometimes we can find obvious contradictions between "logical reasoning" and the real conventions. For example, from logical reasoning we can conclude that a 12-year-old is much more able to give "informed consent" compared with a baby. Thus, a baby needs much more protection against sex compared with the older boy.
But, in (at least German) reality it is not a problem to caress openly the bottom of a naked baby, even to kiss. You can find photos of naked man caressing naked babies in souvenir shops (not sex shops). For an older boy, this is not possible. Why? Convention.
I think, it is obvious that this baoundary is pure convention. But many (if not most) people feel that there is a (often very strong) boundary. Why?
I think the reason is the feeling that sex is dirty. Independent of any sexual revolution, these feelings are strong, caused by a lot of (often unconscious) influence. It is not only the religious right, it is also sex industry with "dirty" porn and the whole society with "dirty" sexual jokes. Nobody can avoid this influence completely.
Tenderness is not connected with such feelings. This leads to a simple distinction: tenderness is what you feel it is "decent", "clean". sex is what you feel it is "dirty", "indecent".