It's probably that they are not able to have children if one is a yokai ╮(￣▽￣)╭
Well, the ending is just to show that Nanami did manage to live like a normal human, but now that she is old enough to make her own decision, she wanted to go back to the shrine and be with both demons and humans like before. They probably can see the others since she did see Mikage in the first chapter. She should still have the land God mark on her forehead since there is no mention about Mikage taking it back. As for Tomoe, it was hinted in the extra that Akura-ou can see youkai after being reincarnated, so I'm guessing Tomoe turning to human doesn't take away his sight to see Youkai.
SPOILER warning tho I guess it's not much of a spoiler but i'm just really confused....
i'm kinda confused about the ending. So they have a kid and then they return? can they see the others again? Does having the kid make them not human again? I'm confused lol