To be clear, theory about what?
Well you alluded to a cultural issue. But broadly, why you think more women are having sex relative to men.
I am not sure I am convinced that more women are actually having sex relative to men. But even if this was true, what possible genetic change in humans could have occurred in the past decade to cause this result?
I know most people here think gender studies is just a big whine-fest about how women are harmed by social pressures, but its also a study of how these systems also affect men adversely as well.
Incels often treat having sex with women as if it was an outcome of a videogame (a concept that is reinforced by society). If one accrues enough "life points", some woman (who by the way has to meet their standards of beauty) is the reward. They feel cheated, like they have the points needed, so why isn't the secret door getting unlocked? So they dig deeper into way the "hack the system" and buy into stuff like "The Game" etc as if it were cheat codes. At some point they project all blame externally, that women are somehow the broken component.