Cobain was apparently pretty monogamous.

I think the bigger issue is the baby, rather than the affair. I'm sure lots of rock stars have wives that are open to letting their spouse fuck other people as long as 1) they keep it out of the press 2) don't bring home an STD and 3) keep the money rolling in. Him getting someone pregnant is creating physical proof that he sleeps around on his wife, which rubs her face and his kids' faces in it, and it also means he was probably fucking without protection.

It's the power dynamic. A person in his position is only going to behave like everyone else thinks is OK because of social norms, but at some point those norms don't apply. Just like no one's surprised that Peter thiel is a suspect for his boyfriend's death. Rich person does rich person thing and people are surprised? I just don't get the surprise part. Grohl has probably had ridiculous amount of people throwing themselves at him his entire life, and we're surprised that at some point he's caved in?

Well, as the song goes, the models will have children, we'll get a divorce, find some more models, everything must run it's course.