From: http://werebereus.deviantart.com/art/Not-Nala-s-Father-211997367 

Reason: Many people who believe this theory usually throw the "Because the Directors said so" reason or the "Mufasa and Scar are the only males in the Pride" reason.



Both are good reason, but they have flaws.



First I will adress the most annoying reason: " Because the Directors said so "



""...So I apologize in advance; but I've been charged, for... ten years

now, with providing to the entire Internet fan community (and believe

me, there are thousands of us) with a definitive answer to the

following question:





"WHO is Nala's father?!"





Full-blown laughter this time, as the question sank in to a couple of

hundred people who had apparently never thought about it before, or who

suddenly remembered they wanted to hear the answer too. Fortunately Rob

Minkoff and Roger Allers were laughing too, ribbing each other, making

little sidelong jibes. Finally, Minkoff looked at me, pointed at

Allers, and said "Roger."





More guffawing ensued. Minkoff and Allers then mentioned how they'd

joked about this during production, tossed around some off-color gags,

and eventually decided that they just "hoped nobody would notice". Heh.

Fat chance, right?





Finally, after talking a little bit about how lions operate in

real-life prides, Minkoff said, sort of muttering into his sleeve, that

the general assumption was that Nala's father was "either Scar or...

Mufasa."



So there we have it, from as official a source as I can imagine."- [link]



As I read this...the laughing, joking, chuckling, and 'guffawing' looks to me like they were just screwing around. And don't forget the 'We hope they didn't notice' part plus mentioning something about 'real prides' which clearly the lions don't operate in. That and they said the 'General assumption'.



To Assume means to "Take something for granted without proof" or "consider something a possibility without really looking into it". Go Check dictionary.com if you don't believe me.

It also means to take on or replace but that has nothing to do with anything here.



Also a GENERAL assumption. Not everyone agreed to this, and unlike Erin Hunter or Steven King's books, who all had a single creator who had all the say-so's, TLK was a Multi-personed Project.



So...most of the people who created the movie went "I guess Mufasa and Scar were the possible fathers"





They never confirmed anything. They perhaps said, MOST LIKELY Mufasa or Scar fathered Nala, but no one came right and said that. Becaused they had no proof and simply considered it.



Only males in the Pride

But the TLK lions don't act like actual lions. They're lions who think with a humans mindset. Also, if Simba and Nala were related, Mufasa and Sarabi would have to be relatives as well. So do Ahadi and Uru, and Mohatu and his mate.



After all, it is a tradition going back generations. But it's clearly not. With so much incest, inbreeding depression would eventually sink in, but no one is smaller than they should be, unfertile, gets sick easily, or die as cubs.



No ones retarded basically(well there's Kiara...). The only one with bad genes are Scar, perhaps one of his parents, and maybe Nuka.