I sympathize with you, but I think you just found the game at the wrong time for you. NTR isn't for everybody, and even within NTR there are many subgenres which may be mutually exclusive for people.
Starmaker Story would generally be considered "Netori" (where you "steal" another person's partner). However, I choose to think of the story from a more wholesome perspective.
Even before the player and Anna start their Starmaker page, Anna isn't happy. She used to be full of life, living a hedonistic, party-filled lifestyle. Then, when she married Joseph... All of that was gone. While Anna gravitated to some parts of her new life, there was a large part of herself she felt she had lost. The player helps her feel wanted again when Joseph doesn't.
The player, Anna, Joseph, and Adrian are all flawed characters, with their own weaknesses and vices to overcome. You, as the player, can help them overcome those vices, or cause them to lean into those vices. It seems like the choices you made caused those characters to lean into those vices.
I am sorry the game had this effect on you; it has icked me out a few times as well. You asked if this is a common experience with the game and I would guess not. The title of the game is "Starmaker Story" and the inciting incident is you agreeing to help Anna make a Starmaker Page behind Joseph's back. I feel that lays out pretty well what kind of game this is (Netori). Most people who would be affected by Netori would probably stop playing.
I'll admit, Starmaker doesn't always get my flavor of NTR right. I want Anna to be sexually liberated and kinky, but only with me. However, many other people play the game and want to corrupt Anna further, sharing her with many other partners. Trying to appease both kinds of players if a difficult balancing act, and sometimes Starmaker is unable to maintain that balance. Sometimes the labels on choices aren't quite clear enough and you see something you didn't expect. I am able to stomach both flavors, but if you can't, it can be distressing.
I am getting the feeling that you were fine with some light Netori, but perhaps not as far as the game goes. Perhaps you were fine with corrupting Anna into leaving Joseph, but perhaps were not okay with her constantly badmouthing him and disrespecting him, or having sex with the player while she is still in a committed relationship with Joseph. If that is the case, I'm sorry things went further than you were comfortable with.
That was a lengthy post, so I will try to summarize. TLDR: 1: All the main characters have their flaws and can work towards correcting them based on player choices. 2: You experience is (anecdotally) not that common, but I sympathize. 3: Starmaker isn't perfect and, like all media, you may find something you didn't expect. Starmaker generally does a good job labeling important choices, but mistakes happen and you might see something you didn't want to see.
I hope things get better for you. Maybe you can return to Starmaker at a better time in the future, but it might not be the game for you right now.