But that's the point. If you couldn't borrow 5x joint salary, or even more in the days of self certification, prices wouldn't have risen so much because people wouldn't have been able to borrow sufficient money to pay higher prices, so they wouldn't have increased so much. Prices are limited by what people are allowed to borrow and can afford to pay.
Allowing higher borrowing has been a main driver in increasing prices. In a way, it wasn't helpful to count both partners salaries when a couple buy together, because that also fuelled house price growth and made it much harder for people to buy on their own.
If they had kept it as only counting one salary in a couple for lending purposes, prices wouldn't have increased so much. It's not anti feminist to say that, just an unintended consequence.
Whether you have a couple or a single person, and regardless of the sexes of the parties involved, if you simply count the highest salary for mortgage lending purposes, it would make it easier for people to buy on their own, because they're not competing against couples, and also makes it that a couple doesn't need both parties to keep working full time to pay the mortgage, so protection against job loss and also makes things easier for those who have children and need to consider childcare costs, SAHP or PT working.
It's been the same for credit cards. When I got my first credit card in the 1980s, the minimum payment was 5% of the balance. Over time that got reduced to 1%, and at one stage, minimum payments were less than the interest charged, so you could pay a tiny amount and the debt kept growing, even if you stopped spending on the card.
Now this didn't make it more affordable to use credit cards, or easier to manage, all it did was have people get into bigger debt before they realised they were in trouble, which tends to happen when people stop being able to afford the minimum payment. If this is 5% and you can afford £100 pm, when you pass this point, you're in £2000 of debt. But if the minimum is 1% you have £10k of debt when you stop being able to afford the £100 minimum payment, and then the interest charged and earned by the lender is far far higher.
The whole system has been set up to trap ordinary people into debt, because that's what makes a lot of money for the 1%.