For ever – or at least for 70 years - make love was many times more common than have sex, at least in the Google Ngrams database of millions of books in American English. And, then — well, you can guess what happened then:
The results are the same with “making” and “having” (you can play with the search here).
Why? What happened? Could it be “the culture”? Zooming in on the period since 1950, preliminary evidence is mixed: