They were not like any other couple; the were the earthly parents of God Incarnate! The wife in this couple was a living, sacred Tabernacle of the Lord, the Ark of the New Covenant, chosen and sanctified to bear the God man. How could St. Joseph even think of having sex with the sacred Ark of the New Covenant? He would have to have been a rash and sacrilegious man to presume to penetrate and impregnate with his own seed the sacred womb which had borne God! And considering Uzzah's terrible fate when he touched the original Ark (II Samuel 6:6-7), Joseph would not have gotten very far had he tried. Yet Scripture portrays him as a reverent man; hardly the type to try something like that.
Take your pick.