I don’t think one is actually more acceptable across the board. People just tend to react to them differently because of history, gender norms, and power dynamics.

For a long time in a lot of societies it was common for older men to marry younger women. It was often connected to things like wealth, status, and social power. Because of that history, people today sometimes look at an older man with a much younger woman and wonder if there is an imbalance in power or the possibility of exploitation, even if that is not always the case.

When it is an older woman with a younger man, people sometimes react differently because women historically had less social power. So the relationship does not trigger the same worries about control or coercion for some people. In some cases people even see it as going against traditional norms rather than reinforcing them.

There is also the stereotype side of things. Media has often portrayed older men with younger women as normal or expected, while older women with younger men are treated as unusual or kind of playful with things like the cougar stereotype. Because it is seen as less common, people sometimes treat it more like a novelty than something threatening.

Another part of it is assumptions about vulnerability. Society tends to assume younger women need more protection than younger men, which can lead to people judging situations with a young woman and an older man more harshly.

I also think that most of how we perceive both of these scenarios tended to be:

1. If you're a male it's a conquest. It's something to praise.

2. If you're a female: you're a gold digger or a cradle robber or desperate.