For a succubus, Mero seemed pretty unperceptive about what was going on.
So he accidentally used the sex doll for its intended purpose of ridding himself of lust so he can do magic better.
I had kinda forgotten about that part so went back on a quick reread. The relevant bits are in chapter 6 when he first gets reacquainted with Reko from his former party. More specifically
pages 9, 10 and 11. Especially 11.
So, yes, indeed. Absolutely accidental.