I think people are being a bit disingenuous with the wide-eyed "who looks down on single people" shtick. Yes of course there are progressive, intelligent people who understand that being single can be a choice, and a good, liberating choice. And thankfully the numbers of these people are growing.
But there are a lot of people in society, probably a majority, who for a variety of reasons they probably don't fully understand, find single people to be objects of either pity, fear or bemusement.
The reasons are different and complex and to list a few I'd say:
a) A hangover from an age led by religious morality where women who were not attached were considered to be either frighteningly loose-moralled or unmarriageable. While most people are not religious any more, there's an echo of this in today's attitude to single people.
b) Convention: a lot of people are pretty basic and aren't capable of much independent thought so they assume anyone who doesn't follow a default path has something suspect about them. And being part of a couple is the default path for most people.
c) Misogyny: Men have done very well for millennia convincing women that they need men to protect them etc and have very successfully conveyed the idea that a woman without a man is a woman who has been rejected by men.
d) Fear that someone is going to change your dynamic: couples sometimes appear to be unsettled by single people for reasons I don't understand.
e) Jealousy: I think a lot of people in bad or suboptimal relationships envy single people their independence and experience this as some sort of implied criticism of their own relationships.
There is definitely still some stigma against single people. Few people are crass enough to come out and say: "Why are you single?" But you can often tell people wonder about it.