Okay, but an au where Allura and Lotor get married because they grew up knowing each other and feel that special spark. Due to some problems he's facing at home, he moves to Altea with Allura.
The relationship is fine. They are friends and love each other and honeslty? They couldn't imagine having to make love to someone else.
But they both feel something is missing. They talk about it often. They try different things, but they can't seem to figure out what it is that's missing.
Then they meet one of their new guards. His name is shiro, and he use to fight as a soldier, but after loosing his arm and having it replaced with a galra prosthetic the captain of the guard in the royal palace offered him a transfer once he was back to fighting shape.
They both know that this is what was missing from their relationship and instantly set about wooing the man.
His blushes and stammers at their subtle advances are cute, but they don't get anywhere. So they try a more direct method. They send for him in the night and when he arrives he is assulted with kisses and praise and gentle hands (not without his consent. They give him all the opportunities to leave if he wishes). They love him and worship him, soon convicing him to join their relationship as an equal and not just an extra in their bed.
Bonus if you would like a four person poly relationship : a half galra named Keith also joins their relationship a year or more later. They have to get a larger bed.