Just caught up to this today. I'd say that the author has foreshadowed a bad/ntr ending. My reasons:
- MC Dad made weak characters. MC's arc is to understand the appeal behind such character by becoming one himself. (my interpretation)
- FMC nasty sister said FMC is the "betrayer".
- Lot's of NTR teasing since the beginning.
- A NTR dojin who's main character became canon. (that's actually pretty awesome)
- The Teacher breaking all boundaries. (sex is now on the table)
Mangaka could just be teasing and it will be a happy(ish) end. Or we could go full Guilty Circle. I'm good with either, I am not really attached to the MC. I don't identify very much with him. FMC isn't anything special either. Her personality and character seem to change chapter to chapter. One second she is an introvert "wrap" fetish perv and the next she is a innocent perfect student. Her only endearing quality is being hot. Teacher is just trash, scornful used goods, to put it indelicately.