*An adult being the only earner and just about making it will soon get resentful of the other.
This isn't necessarily true. I see a lot if this on Mumsnet, a lot of women urging other women to never trust a man to support them, presumably due to bad experiences. It makes me a bit sad because it's such a bleak view and because I know so many decent, good, hardworking men who look after their wives and children.
My husband for one is proud to support me and our son as the sole breadwinner and doesn't resent me as some sort of freeloader, but rather, appreciates the contribution I make in raising our child.*
Nothing to do with a bad experience, a quick search on MN will show posts of resentment, control and how women are left with nothing after a split.
For me, it's not about having a bleak view. It's about pride, partnerships and a measure of protection. If DH falls ill or loses his job, I can still maintain the household. We share everything, the financial side, parenting and house stuff.
I'd hate to be reliant on a man for my every need when I'm very capable of working. I don't need someone else to buy my food, heat, tampax etc as have my own earnings, likewise I can treat DH or buy him gifts and pay rather than him actually paying from his salary. I don't want my DD to think she can opt out of working simply because she was born female.
One income means all the eggs in one basket and usually means no security if things go wrong.