The person who chooses the life of consecrated celibacy over the life of marriage, has indeed chosen the better part. But that does not make marriage or the marital act sinful. As St. Paul says, "If you have married, you have not sinned, and if a virgin marries, she does not sin." (1 Cor 7:28)
-A8
St. Augustine went between the extremes as it were. And yes, he did say that martial sex was a sin. He also changed that view. But the view that even martial sex is a sin was pretty widespread for quite awhile.