Well firstly if you are good to the gf you can avoid ntr. I avoided it and she even gave me bj in classroom, but another things bothers me a little because judging from the conversation with joshua at the diner it means that milf was a porn actress in the past ;v
even if ur nice to gf she stills act too weird and stills act like she is cheating, thats why you should be carefull, until now it seems "safe" in a way that we dont see any ntr unless we are an ass to gf (though i wont say is entirely safe because is weird how she tells the mc she is being training giving deepthroats, how she apologize to mc and says she is no good at all to him....and many other things )
maybe you
@UncleVT can send the dev an email asking this exactly question (does the gf cheats even if ur good to her?) so we dont get any surprise in future updates xD?