About the time the speculation of the Immaculate Conception was being developed, sex of any sort was viewed as bad. Or at least not a good thing. Augustine himself said that sex, even between a man and wife, is at best a venal sin and possibly a mortal one (if you only lusted after the spouse). There were a lot of reasons for this, much of it has to do with the influence of Plato's philosphy.
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
Any evidence marital was viewed as bad by the Catholics? What you describe is a Puritan and a Gnostic belief, and St. Paul taught against it:
doctrines of devils, speaking lies in hypocrisy, and having their conscience seared, forbidding to marry ... (1 Tim. 4:2f)