It is inconceivable because Mary already had the joy of full union with God which is merely foreshadowed in the joy of an ordinary spousal union. It would not have been sinful, perhaps and legalistically speaking, for her to carnally enjoy the marital union with Joseph, merely absurd. Quoting myself:
Mary had arrived to the unitive and procreative marriage to the Holy Ghost abundantly. Sinless, she was united to Him at all times. Her body gave birth to the perfect new man. She was walking in paradise all her life. The hormonal urgings a lesser woman goes by had no purpose in her; if she had them they were not significant and probably not noticeable to her enlightened mind.
Blasphemy!
Luke 1:46 And Mary said, My soul doth magnify the Lord,
47 And my spirit hath rejoiced in God my Saviour.