Porn has existed for thousands of years.
How do you know people are having less and less real sex, and is that necessarily a bad thing?
Discussion
There are a number of factors contributing to a decline in population. In the US, I would argue fiat economics plays a larger role than porn.
In some form it existed, but I think they never called in pornography. They saw it as "nude art" or so, e.g. look at the old "pictures" there at Wikipedia: https://en.wikipedia.org/wiki/Pornography . As I said, I found it a bit odd, that these art works are under the same article as today's "modern porn", like from Brazzers (all the same boring thing, no matter what you watch).