Treating matrimony and motherhood as if the very notion of marital relations between herself and her lawful husband Joseph was somehow sinful, viewing it as fornication I suppose, dishonors Mary. If she actually were sinless, marital relations with her husband would not have made a sinner of her. It’s all just so convoluted.
I've encountered that mentality about Mary having sex with Joseph in marriage from Catholics before. They do equate her sinlessness with her perpetual virginity.
Catholics in general have some weird ideas about sex, like it's somehow bad. It shows up in the Mary stuff and the way they laud virginity and celibacy as being a superior lifestyle and more honorable and pleasing to God, somehow.