I won't ruin your thoughts but do you really think a manga like this would imply characters had sex?
No it will explicitly said it. A lot of jokes have sexual innuendos. Hell, Inugami is not hiding her M tendency at all. Ushikawa had been throwing innuendosa lot of times. I don't think imply two characters had sex off screen is too farfetch. Also remember that this 2 characters had call Nekoyama's sister the super Nympho.
Mist likely is because it became a mess. The story loss it's originally concept and had too many characters that readers were barely able to remember after awhile.
I can get this already. It was fun with the 3 main cast, the Animal club and Sarutobu/Torikai showing up from time to time but characters keep piling up regularly and it's difficult to remember who is who and most importantly to keep the focus on the main cast.