Big difference with Marie-Louise, aside from the potential of statutory rape, being under the age of consent, her actions would rarely reach the point of a trial/hearing. Especially if she were to make a deal with the mother of the further underaged minor. I didn't check Nelson, Rich, or Stu's IDs, but they may very well already be 18. If so, Marie's fun-filled evening would not be illegal for her.
I can't say for sure, but unless the fetus was in process of coming into the world, I believe sexual activity is not harmful at any other time during a pregnancy.