I am single and choose to travel in Premium Economy or Upper/Biz. When I was part of a couple, I did the same.
This came about because family live in South Africa and I go there fairly often and, through biz travel in the late 80s and 90s, I got far too used to going Upper/biz!! I do not do much biz travelling these days as I am in a different line of work.
I have decided that I will not travel lonmg haul for more than 6 hours (approx LON~NYC) in Y. I use money and miles to upgrade. Of course, if there was a family emergency and I did not have the money I would go in 'Y' but it is not my choice.
Income is certainly not the only factor.