Aside from how they act, Darkness, Megumin, and especially Aqua are supposed to be very beautiful in that world. Aqua specifically, because she has a divine presence that makes her more beautiful. So the receptionist is nothing new.
Despite how he's on and off with megumin at times they are in a real relationship and even if it's by the thinnest margin it's still very much real compared his relationship to darkness where he has made it more clear to stay friends. Plus Kazuma does these kind of things as a joke + succubus joint is an option whenever he feels the need to let go.
So all in all it was just a bit of manipulation for the manipulator of many.
Consider that Kazuma lives with three very beautiful landmines already.
I completely understand him not wanting anything to do with a potential fourth.