Yeah, well, you got me. Happy?
In none of the translations I have looked at, does it say anything about "love slaves". That seems to be something you added in for effect. The translation you cut and pasted could mean that, but it said "keep for yourselves". This could mean to keep them in some other form of servitude. Furthermore, as you will notice, the passage in Numbers 31 that you quoted is from Moses, not God. Moses is human, and therefore subject to the sinful nature and desires of man.