That's it. It's not some moral assessment on people who ask for help paying the mortgage so much as that arrangement is encouraged to be put into rental agreements for understandable reasons. However the laws governing those rental agreements set the expectation for a very exploitative relationship.
Ex. “you don't wanna pay for someone else's mortgage” is not a saying for no reason. It's very immoral to have someone paying for your future housing security often for years and years only for them to be shit out of luck for their future housing. It wouldn't be much trouble to change housing rights laws so that everyone is always guaranteed good retirement housing or housing if disabled or just guaranteed housing (the only moral option tbh) but NOPE the law in the U.S. where I live was written by a small class of capitalist who really love to exploit working people and poor people.
Ex. Rent is not at all tied to income. And never has been for a lot of poor folk and in a lot of places but it's really really bad right now in the U.S. this exasperates the problem in the 1st example and also gives a landlord a whole lot more power over renters who are suddenly a lot more vulnerable. And besides the terms of the contract being unfair since it don't guarantee future housing for the renter landlords in reality overwhelmingly take advantage and don't do repairs or jack up the rent because there's not many other options.
So yeah. being a landlord is immoral. And if it wasn't like OK tell your spouse/partner you only want your name on the mortgage if/when you buy a house and see how that goes