Are Meliodas and Elizabeth still in love after he becomes the demon king?
The reason he is cold towards Elizabeth is because


   the demon king removes his emotions and imprisons them in Purgatory when he is killed by Estarossa and the other Commandments. The Meliodas that returns to life is without feelings, and is just keeping his promise.


However, in the latest arcs (Manga spoilers; not in the anime yet)


   Ban journeys to Purgatory and successfully manages to prison break Meliodas' feelings. Though they get split up on the way out, it is clear that both of them managed to escape. Thus, with his feelings restored, Meliodas should love Elizabeth again as before.