I have to give the manga & light novel credit for how it's gone into the full harem route. Lefi's being incredibly sensible, which fits her and she's right. She and Yuki are going to be together for a very, very long time, but Nell especially is not going to be around long (by Lefi & Yuki's standards, but even human ones likely), so why not make her happy in the limited time she has left?
But that logic falls apart a bit with some of the other girls. Iruna's a vampire, so she's probably going to live much longer than the others, but probably not as long as Lefi & Yuki. No telling how long the slime girl will live, but the sword avatar is probably going to be around as long as Lefi & Yuki are. I think she's more of a daughter, but then again, I thought Iruna was as well. Making your sword your wife seems awkward, no matter how cute a human avatar it has.