I believe, though feel free to correct me if I am wrong here, that at one point the Church’s practice (maybe if not doctrine) was to only condone sexual relations for procreation. Then (I think at Vatican II, but again, correct me if I’m wrong) the Church officially stated that sex had two aspects, unitive and procreative, and that no sexual acts could be morally justified without the presence of both within marriage.
I don’t know what Saint wrote this, not to say that they are wrong, but I am just putting this out there. Many Saints wrote many books and 99% of what is said in those books are true and will help you lead a moral life. That being said, the Saints do not speak with the infallibility of the Pope, so it could be possible that this particular Saint was wrong about this particular issue.