Honestly, in the 1980s and 1990s getting married was just what we did. There were some tax breaks and it made life easier for mortgages if you were married. I also think that we expected less in terms of materialism and our homes were largely furnished with the cast offs from the homes of mum and dad.
We didn't expect to have fabulous careers from the get-go, very few people went to university, a lot went to work at 16 and a lot did post school technical training and went to work at 17/18/19. By the time we all reached our early 20s we were working and beginning to climb the career ladder. So at 21/22 we'd been working for between 3-6 years and probably well into our second or third jobs by then.
I also think that there was less disparity by getting together younger. A lot of couples, like us, started with nothing and built everything together. These days, by getting married later everyone is comparing monetary wealth and assets and ensuring bills are split evenly, etc.
Weddings were also not the ridiculous affairs they are today. Receptions were pub function rooms or town halls, or even just a sit down wedding breakfast at a restaurant, ceremony was church or registry office, those were the options.
I don't honestly think anyone "got married to have sex" and certainly in my circle no one would have batted an eyelid at people living together even without marriage on the horizon. We had family planning clinics, didn't need to be married to go on the pill, condoms were free and this was early 1980s and totally nonjudgmental.
I grew up in London so I can only talk about London standards but my parents thought we should live together first and my grandmother also didn't bat an eyelid and she was born in 1903.
Basically, getting married was a commitment that we all embraced. It was the next step followed by children. However, in those days you couldn't get maternity leave until you'd been working somewhere for 2 years so everything had to be timed cleverly for maximum benefits.
I do agree that single parent families were not the norm and I do know the more than one couple who "had to get married" but it seldom seemed to work out well. I also remember one girl, bless her, whose mother would not let her go on the pill and she had at least three abortions that I knew of before she was 18 so, yes, some parents had principles that were stupid in the extreme.