[quote="Elton John"]Which begs the question, is Simba the father of tiifu and zuri?[/quote]
Obviously not. Like TTB-Kun said there has to be other Male Lions in The Pride Lands. Makini proved this in "The Rise of Scar" when she stated that Fuli was the first Female to ever serve in The Guard.
Now with that and what we saw in "Return of the Roar". With Simba telling Kion to choose The Pride Lands Bravest, Fastest, Strongest, and Keenest of Sight. To form his Lion Guard. Then when he saw Kion's team (after chewing Kion out) he told him that there were plenty of good Lions for The Guard.
"In Lions of the Outlands" Kion asked Kovu and Nuka. Why don't you live in The Pride Lands? Not Why aren't you part of My Dad's pride?
This would indicate that other Male Lions live in The Pride Lands, but are not part of Simba's Pride. Now I have never heard of A Lion Pride Allowing Male Lodgers to live on their land, but this is Disney. They have at least very strongly hinted that other Male Lions live in The Pride Lands. Albeit apart from Simba's Pride, presumably not to challenge Simba's Kingship. Obviously Simba's Lionesses are allowed to...Uh...associate with these Lodgers. As evidenced by the existence of Tiifu and Zuri.
I doubt the show will ever address the exact nature of this arrangement, however If the situation were dire enough, I would not be surprised if these Male Lodgers could be called on to assist Simba's Pride. (Especially Tiifu and Zuri's Father/Fathers).