Alot of people still believe in this theory where Nala's father is either Mufasa or Scar because it apparently "follows real lion behavior".

Those thinking that incest in lions is a purely natural occurence, why don't y'all take a look at these articles?

This one's about zoo inbreeding, and this one's about the decreasing lion population in the wild, leading to incest.

So I guess there IS lion incest in the wild but not without human intervention such as harming the conservation status. Thing is, The Lion King is a world without humans and with personified animals, two things that prevent incest. 

I hate how our own kind tends to blame their own doing on nature.

If you still want to go along with incest theories, that's completely fine by me. Just as long as it's not glorified for any reason and is still antagonized. 

Besides, we all know the answer to this very age old question:

Who is Nala's dad? And the answer is: 

NO DAD. MOMS ONLY.

(Obvious joke is obvious, still a cute headcanon of mine.)

(Fanart by yours truly.)

In all seriousness though, weither you like The Lion Guard or not, Nala's father has been officially recognized as this nameless lion mentionned by Rafiki, who is said to be a carbon copy of his grandson Kion, and got saved by the Lion Guard as a cub when he fell from a tree. What happened to him is up to interpretation once again. Also no, he has no blood relation to Mufasa or Scar.

(Here's Rafiki's painted portrayal of Nala's father)

So yeah. Feel free to go along with any headcanon that you like, just wanted to put this out there.