Headcanon~
Alastor allows his tail to be shown near Lucifer, along with the personality drop.
With everyone else he's in the persona and never shows his tail, and really never shows anything at all.
It would take a lot for him to love anyone else
But I'm questioning if it were possible.
I'm not sure if Lucifer and Alastor are actually in a closed relationship with each other, or more of an on and off and Lucifer goes and screws anyone he wants and Alastor just isn't interested anything with anyone.