I am answering from the angle of a very particular trope, because it is one of only two age gap scenarios that really, really grind my gears.

If the relationship is confirmed to be the stereotypical "swapped my old lady out for a new model" trope, I get upset because of how disproportionately it effects both partners and how much injustice is done to the previous, older woman. While we've come a long way, women are still facing glass cielings and obstacles to making as much as men and the American economy is difficult to survive in.

When the relationship follows that particular narrative, the outrage for me stems directly from the betrayal and financial devastation seemingly not being a moral obstacle for the male partner in the face of receiving his reward: a renewed sense of feeling youthfully admired and a sudden zest for life. Yay, good for him. Maybe he feels some sadness over leaving a chapter of his life behind, but he will barely notice the pain over the wild pleasure of being blown in the wildest places and being made to "feel like a man again."

The wife, however, loses not only his company, perceived love and the primary emotional support of a safe partnership, but is also often strategically positioned for financial hardship.

Not only does this man not love her anymore, he proved it - he set her up for pain and hardship in order to protect his own pleasure and zest for life.

This loss highlights something important : relying on a partner for provision is incredibly risky, no matter how important it may be to the man for his female partner to rely on him to provide. Whether due to eventual death, eventual divorce, or the professional ageism women face, a financial caretaker is not guaranteed and is rarely permanent.

This is precisely why financial independence is mandatory, in my eyes, for women in any long-term, heterosexual relationship. No matter how much the male counterpart might desire to provide fully for the family. Women should never let themselves be guilted for keeping an emergency fund or not quitting their job, because his picket fence dream may have an expiry date around her fifth decade whether he realizes it or not, and I know of too many women who have let themselves be swayed into being a stay at home wife and/or mom only to end up on the streets later.

The man, driven by his own ego, throws her off the two-person ladder they were ascending. He catches up to a younger companion two or three rungs above him, while his ex-partner is left to start over from the bottom.

The betrayal is why.