I think I know what you were trying to say OP, but I think you've worded it a bit clumsily, and are also coming from slightly the wrong angle.
The cultural 'norm' in Britain is for people to purchase (via mortgage) their own home once they are in an established career. In a lot of mainland Europe that simply isn't true - owning is the minority compare to renting, which is the cultural norm there - but the fact remains that buying here is what we, either overtly or subliminally, believe to be part of adult life.
I suspect your post was reflecting on the inequality between salaries and housing prices? Because yes, it is now much more difficult to afford a mortgage on a single salary, so you wonder why people think they should be able to - but the fact remains that, considering our cultural 'norm', single people should be able to afford a one bedroom flat on their income alone.
The comparison with 50/60 years ago and people only expecting to buy a house when in a couple of pointless - more people tended to marry much younger, and then there was only one income because mothers stayed at home with the children (although not always the case: my grandparents were always social housing tenants, both worked - including two jobs each at one point to make finances work). So they were still single income households, albeit housing however many individuals.
There is a massive disparity between income levels and housing costs in this country, and quite often people are paying more to rent than they would with a mortgage, but can't get a mortgage because of their annual salary...it's a viscous circle. Plus, there is the uncertainty with renting - the only secure tenancies are social housing of some form, and they are comparatively few.
Everybody deserves affordable, secure housing - whether it's rented or bought, there needs to be enough homes for people to live in. And for a lot of people, that looks like buying not renting. Single people shouldn't be at a detriment because of their single status.
(I could be way off, and you were actually just trying to provoke an argument, and you genuinely believe single people should resign themselves to house shares/bedsits/studios until they die)