When did Mary and Joseph learn that they were never supposed to have marital relations, according to Roman Catholic doctrine?

Roman Catholic doctrine teaches that Mary remained a virgin her entire life, even after marrying Joseph. It would seem that Joseph and Mary, at their betrothal, had likely expected to have, and anticipated having marital relations with each other in marriage. So, according to Roman Catholic teaching, when and how did Mary and Joseph learn that marital relations would be denied them? Did they just never desire that, or did they specifically have to battle the temptation? It admittedly seems strange to me that this would be withheld from them. Is it even right to say they were not supposed to have marital relations? Or should it be said that they never even desired that?