This reaction wants to redefine adulthood as post 25

It's even more than that, it wants to make adulthood some kind of sliding window where the age of the older partner defines how "adult" and "capable of making decisions" we see the younger partner, and the older a person gets the more people at the lower end of the age range get excluded for them from this fictional adulthood. For example: 60 and 30 would also be seen as inappropriate.

Now it's perfectly normal for younger people not to find much older people attractive or suitable to have a relationship with and vice versa, and they may even find the idea repulsive, but this is still a personal preference. It's probably even the preference of the majority of people, but that does not mean we should take away the agency of adults to choose their partners when they have a different, non-conforming preference. At that point it has nothing to do anymore with protecting vulnerable people from predators, but about imposing your own preferences and dating standards on other people, and you're quite right in calling it out for the neo-puritanical and conservative thinking that it is.