>>70811 (OP)
Iv thought about a world where everyone wears diapers, but have never thought of an underwear fetish in said world or a desire to be continent. Where the roles were totally reversed.
I can imagine an almost believable scenario that is getting sort of close to this though.
Imagine a world in the future where mankind has finally managed to break free of the Earth. We are a truly space-fairing species. It is not at all unusual for people to be born and grow up in space. In fact, it is actually far more common to spend the majority of your childhood in space than it is to do so on a gravitational body, since the population living in space is much higher than that of any planetary colony or even Earth itself. Our advanced technology has solved most of the problems associated with living in a weightless environment. Our mussels and bones no longer waste away to nothing outside of a gravitational pull. The radiation problem was solved too. But as much as technology has improved since the 21st century, some new problems emerged to take their place.
One of these problems was that of potty training in a weightless environment. On a planet, "accidents" are of little real consequence. The worst that can happen as a result of wet or soiled clothing is making a mess on the floor or furniture, and also causing some degree of embarrassment to the person. In space, there is a critical difference: There is no gravity to pull the liquid to the ground. Instead, it floats around and gets everywhere. It is then not a matter of little real consequence as it is on a planet to wet one's pants. From a sanitary viewpoint, this is a disastrous occasion in space for obvious reasons. From a technical point of view, liquid can short out electronics or cause all sorts of other damage.
For this reason, it was of the utmost importance that uncontained accidents be absolutely minimized in space. With potty training children, it is of course impossible to avoid accidents, but considerably less so to avoid uncontained accidents.
Additionally, it is much harder to get a handle on the use of a zero gravity toilet as a 3 year old than it is to use a toilet on a planet.
The end result: Potty training in space generally starts a year or so later, and tends to take longer than on a planet. Even after a child has mostly figured it out, they continue to wear training pants for many years to come. Over time, it becomes normal for older and older children to continue wearing training pants, both as a "just in case" precaution and because it is just more socially acceptable to do so. Eventually, it is not unheard of to continue wearing them even into adulthood.