itch.io

It may be a bug since you are infusing magic though there are legitimate ways to skip the sage with magic in the form of killing the fairy and making a magic potion so I cannot be certain. 

Is there a walkthrough on how to unlock certain sex scenes? I believe I am at the end of the content but I am missing several scenes.

There does not seem to be any such encounters after you encounter her and she definitely stops seeing others after MC seduces her. 

NTR is entirely optional. Luna does have sex with another guy but that is a scene you get when you first meet her so it isn't exactly NTR

If you are worried you can disable that in the settings.

Pixies pregnancies are super quick due to artifact energy.  So you cannot have pregnant sex with her.