Every survey of whom? Did it include people married for over 40 years? Most of the "surveys" you find are conclusion-driven to target an audience for an article, something like "how are people finding their SO's today"?
It's going to be hard to come to consensus on a number because it changes over time. Are we talking about all married people today, people of a certain demographic (18-34, etc)? During what period?
My source for the ~50% number is "common knowledge in the 80's and 90's". Anything you find by googling right now is going to have definitional problems. I'm not aware of something like a US Census that would be definitive.
What is your source for this figure? I just googled around and every survey appeared to put in the 15-20% range.