I'd say, with the natural force-talent Rey has shown, she could very likely be the daughter of two force sensitive people. That can be Luke plus an still unkown female apprentice, or two unkown and maybe new characters.
I can't see Luke putting on the restrictions of the old Jedi order, so it's possible that he took in students that were around his age when Kenobi started his training, and that he won't forbid his students to have relationships.
Did Obi-Wan ever tell him that Jedi knights are not supposed to have relationships?
I would imagine Obi-Wan/Yoda/Anakin/Qui-Gon have told Luke all about what happened back in the day.
I suppose it goes back to your interpretation of the "balance" concept and the prophecy.
Some assumed that Anakin had fulfilled his role in the prophecy by equalizing the light and dark sides (Obi-Wan and Yoda vs Vader and Sidious), whereas I have heard that Lucas said in interviews that being imbalanced towards the light was the natural order.
What an interesting twist it would be if the force was causing these cosmic events to thin the herd because the optimal status for the universe is a very small number of powerful/influential/trained force users of both sides and not a bunch of people running around actively using powers constantly.
I lean more towards what Lucas described. The Light side is sustainable and not self-destructive, whereas the Dark side (and especially the Sith) will eventually lead to its practitioners' destruction.