This makes sense.
I can see the point of view on both sides. I think when living together then proportional or pooling money works best, especially when there are big gaps in income, otherwise you can't afford to do things together in a meaningful way.
At the same time, the idea of pooling or going proportional can be hard for higher earners because it just means they are paying more than normal for everything (e.g. going out, eating out, going on holiday, clothes, food etc). Because - for example - the high earner is paying 100% of their own costs plus 50% of someone else's costs.
I think when you have no children together, and when you have had past breakups (I.e. divorce, or from cohabiting for a long time) then the idea of subsidising a low earner is even more difficult to swallow - they did it before and that didn't work. This is on top of feelings of "if [lower earner] is struggling, they could get a better job. What would [lower earner] do if I wasn't here".
Obviously, you can have your own personal costs that complicate things, for example for your own adult children. If your partner pays £500 extra for the holiday and then you gift your child £500, does that mean he has effectively given your child £500? Does he want to do this, especially when to pay extra for the holiday he had to deduct £500 from his regular savings, which would be his children's inheritance.
It also sounds like your partner likes the idea of saving, and generally being frugal. So the idea of someone else to pay for (after paying for a divorce) is likely to be even more annoying.
I don't think any one is "right", I just think it's wildly different perspectives and is a element of compatability. At the same time, I don't know how relationships with large income disparity can work long term if the higher earner isn't willing to pay more eventually. At the same time, paying for someone else means being financially irresponsible, especially if you break up next [week, month, year].
I also don't think this is talked about much because most people assume a nuclear and traditional family of no divorce, shared children, man earns more, woman has sacrificed career for him - of course he should pay more. And then a bunch of norms have been created around these things (pooling finances, proportional income). Along with descriptions like men should be generous and providers for women. So if a man doesn't pay more, it just makes him look bad, when in reality he probably has a bunch of valid concerns.