I once had a young colleague who was ... sharing her housing cost 50/50 with her boyfriend who earned a lot more than her. When I was young and my HB and I were not married and still had separate bank accounts, we used to share the cost based on our incomes. He paid 50% more than I did.
[...]
Gotta keep in shape so the douchebag boyfriend doesn't leave her?
I don't think we have enough information to determine whether the boyfriend is a "douchebag". Splitting shared costs 50/50 sounds potentially fair to me, in certain circumstances, provided among other things that both people have the same say in choosing to incur the costs.
Splitting costs based on income is potentially highly problematic because
income is fleeting.
Suppose one partner is earning 5 times as much as the other partner for a while, but then takes a year off work. During that year, their income is now 0, so should they pay nothing toward the rent? As can be seen, splitting expenses based on income would create a perverse incentive to artificially lower one's income. The obvious solution is then to "impute" income based on what the person
could be earning, but then that has the effect of seriously limiting freedom; for example, a partner could not accept a lower-paying job that they would rather do without then having to pay a disproportionate ratio of the expenses because the higher level of income would still be imputed to them.
That said, the parties to a relationship are free to negotiate
whatever terms they wish, even if those terms create skewed incentives and set the relationship up for financial failure. By the same token, we shouldn't label somebody a "douchebag" just because they negotiated a different arrangement then you would have yourself. And incidentally, we don't know that it was the boyfriend who proposed or insisted on this term of the arrangement. The "colleague" mentioned in the story might be the lower-income partner today, but maybe she realises that income is fleeting and understands that in the future, her boyfriend might have a lower level of income than she does, and when the positions are revered, she doesn't want to pay more than half, and hence she negotiated equal cost sharing today even though it's currently not favourable to her (because she expects it to be the best arrangement for the long term).