Thank you! 
I don’t think many other posters here disagree with you. There is real life, where many would think like that… and then there’s King of the Hill and Dale. Of what little I watched of KOTH, I took that Dale has weird views. He is not based in the same reality the other characters are.
This is someone whose wife got pregnant while he was not around. I mean, in the real world, most guys would at least gone :dubious: . Nope, not Dale… That is probably why nobody will tell him.
That is true. I remember the ep where Dale thinks Joseph is an alien child because he was away when Nancy got pregnant. And Joseph comes up with the theory that aliens took Dale’s sperm and implanted it into Nancy. I think Bobby said something like, “That Joseph, he really is his father’s son.” It was a really sweet moment. “Of Mice and Little Green Men.”
I’m also not sure I would tell myself if I knew. I might, as was pointed out, encourage Nancy to tell, but I’d feel weird. Being the bearer of bad news can really suck.
As some people have noted, it’s not like anyone has any secret knowledge of the situation. Dale is seeing the same thing that everyone else is seeing. So nobody could tell him anything he isn’t already aware of. So it’s not a question of telling Dale what’s going on behind his back - it’s a question of explaining to him what’s going on right in front of him. And while you might have an obligation to tell a friend something they don’t know, I don’t think you have the same obligation to tell them something they don’t understand.