No but the fact that he conceived an heir with the husband means they were doing it.
Besides a marriage is a marriage. It’s a contract, and a contract is an exchange. You can’t just use someone and toss them aside. He should still be treating Nathaniel as his spouse regardless of the purpose of the concubine.
Yes he's having sex with BOTH of them to make babies. That's literally why they are there. I wouldn't be shocked if Nathaniel was the one who had MC and his baby killed in first timeline out of jealousy
He 100% is the one who killed them. He seems like a total dick.
The only reason I don't feel too bad for Nathaniel is he was also just sent to be a baby maker, just as Chris was. Maybe Nate was just too "interested" in the king xD. The way they described the marriage didn't sound like it was out of love just the need for an heir. I could be wrong tho ofc we haven't seen the king interact with his husband at all