My thoughts

Now hear me out...this my own head canon from the little info we got....

Maybe when the Mc asked the demon lord if he really wanted to hear his answer he was like, "Tbh I want to go back home, I have unfinished business/miss home, but I love you." And that's why they had a passionate night where all they did was call out each others names because they may never see each other again. Which lead to the baby.

Mc tries to go back on the blood red moon just to see if it works. I imagine a heartfelt goodbye and mc wishing for them to see eachother again someday, asking if its real to come bring him back. Mc does what he has to do back in his real world and goes to buy the new game release of the demon lord. Maybe its a trigger for going into that world, idk but he's there for a game. So the demon lord waited till the next blood red moon to try and leave himself to go after hime and that's when they meet again. The demon lord is there to bring him back so that's why the person caring for their child is like, "I hope they come back soon" (●'◡'●)ﾉ

Idk if this makes sense but its what I thought, even if I was confused at first. This interpretation though makes the ending a bit happier (=・ω・=)