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.