I wouldn't say never considering how much of a slut Airi is. It also seems like the three of them are becoming genuinely close friends since they hang out together frequently. I'd argue it's only a matter of time before Airi wants to have a 3P and Yuu-kun obliges since he seems pretty cool with indulging her sexual fetishes (except Anal the absolute coward). Since they don't really interact with any other characters the natural choice would be Kou since he seems pretty harmless all things considered and they have gotten closer/more comfortable around each other and their antics.
That said, I do agree it is unlikely since the joke seems to be that while Airi is a slut she's a faithful slut and the joke about Kou being the ineffectual NTR "chad" would be ruined if he ever got to seal the deal.