Responses
I’m pretty sure Curtis just said that out of pride. He’s being stubborn over the fact that he tried to get her pregnant but failed. I think Curtis, as a male, is equating her pregnancy as “proof of manliness”. It’s as if him failing to get her pregnant would somehow make him less of a worthy male. So he’s refusing to accept it.
The other males, whom she’s mated to, said that the baby doesn’t belong to Curtis because they can smell it being Muir’s.

That can't be Muir's right? What happened with Curtis telling everyone that she was pregnant before Muir even came to be a partner? But nobody believed him. It must be Curtis cub. But poor Muir if it is truly Curtis. He looks so happy to be a future dad.