“But individually, why should another person fund your lifestyle?” Because ESPECIALLY when you have kids it’s not your (individual) lifestyle it’s your (joint/family) lifestyle.
You’re coming at this from a perspective of 2 adults earning roughly the same, both working full time (I think?) and neither irresponsible in any way with money.
But when one earns significantly more than the other, more than likely possible due to the other party being part-time/sahp, and/or one party is a spendthrift/gambler/addict then there IS an issue of balancing things so they’re genuinely fair.
It’s not fair or morally right eg for a high earning husband with a sahm wife who enables him to have a family AND work long hours (often at short notice), travel for work etc to have hundreds even thousands as spending money while she’s left scraping pennies together to buy anything but basic essentials. Why should a wife (or indeed husband or partner) in that situation go without because they have jointly decided that this is how they will manage their family organisation?
It’s a LONG time since I was married but prior to the divorce my ex was fair and so was I, because at various points through the marriage sometimes I earned more, sometimes he earned more, and at the end I wasn’t earning at all.
We had a joint account (admittedly I regretted that when we split for different reasons) from the beginning, both had our wages put in it and after some hiccups at the beginning (he was a bit of a spendthrift and of a mindset he wasn’t stressed about getting in debt - until I educated him about interest rates and bank fees!) we set a budget such that bills were covered, a little set aside for savings and what was left divided between us equally as spending money we could spend as we chose - one of the hiccups in the early days was he THOUGHT I was spending more on personal luxuries than he was, but when I sat him down with a bank statement and showed him that actually he spent (iirc) 3 x more than I did on personal stuff (and he certainly wasn’t earning 3 x more!) he was shocked and suitably apologetic.
His financial literacy when we first married was appalling!
I was a sahm at the end and at that point we still both viewed all income as “our” money (we got child benefit and a small amount of tax credits too) and continued with bills covered, savings covered and same amount each for spends.
Certainly as far as the law is concerned you are financially joined and all income and most assets are joint.