Marriage is in remorseless decline. This chart doesn't do justice to the problem. It only measures the number of marriages each year. With a growing population, it would be reasonable to expect an increasing number of marriages. However, that is not what is happening.
There is at least one silver lining. The number of divorces is also falling. After all, you can't get divorced if you aren't actually married.
I don't have much of an explanation for the decline of marriage. However, I think a friend of mine offered a clue when she went to a hairdressers, just before her big day at the altar. She told her stylist that she was getting married, who commented "how posh!".
Yes, getting married is now only for the middle classes. Everyone else puts up with a casual and non-committal arrangement known as co-habitation.