1. Seline was jealous of the protagonist of his girlfriend, doubted that he loved her.
2. She started dating Jason only to hurt MC and make him jealous.
3. In one of the dialogues, Jason complained to MC that Seline avoids intimacy with him.
4. In the story, when Jason catches MC and Seline having sex, it becomes clear that Jason is a spineless cuckold. Nobody asked his permission. He was simply presented with a fact.
5. And the main thing. Seline loved and loves only MC.
DrFree
There are a couple of scenes with the family of the protagonist's friend.
They are not big, but they cannot be avoided.
It is justified by the plot (the family is controlled by the devil / succubus/lust)