He kind of doesn't have a choice in the matter.Just wife her, bro!
Lefi just needs to be assured that she's is the indisputable Queen of the Dungeon.Really now ?..
He marries Lefi off-screen in some flashback, while we have to suffer through the Demon World Arc, just for him to come back have some weird attitude that's pretty much a 180° towards before the arc (Feels less like him being happy about seeing her again and more like "whatever", the opposite of before the arc) and now they introduce some random ass harem polygamy plot, which lefi is suddenly completely okay with (yet again complete 180°)
Not sure what you're trying to base it on, but usually it's most common that Dragons only have a singular mate in most stories including them ô_oLefi just needs to be assured that she's is the indisputable Queen of the Dungeon.
Think about it this way. If you're bad-ass enough to wife one of the strongest dragons in the entire world, then it'd be normal in terms of draconic culture (might makes right) that there'd be a few other "mates" involved.
Just so long as they all recognize that Lefi's the boss, then all is well.