I know it has been denied officially that Nuka is Scar`s son, but here are my theories.
I do quite like the idea of Nuka being the only cub of Scar, because he fails so bad.

It would also explain some of the resentment Nuka has for his younger brother. My theory is that although Nuka was born to Scar and Zira (assuming for a moment that Scar mated with several lionesses in the pride in order to secure his genes and produce heirs.. NO, that does not include Nala..) Scar chose a different cub to follow in his pawprints - Kovu. We`re talking theories here, but Kovu is not Scar`s on. Maybe Scar thought he saw something in Kovu that Nuka was lacking, maybe he thought Kovu looked like he`d grow up to be a strong, cruel leader? Zira does mention that Scar handpicked him to be his succsessor.
Another of my theories: Vitani and Kovu are littermates, but their father(s) is a rogue, none of them are Scar`s cubs.
So my THEORY is that Nuka, Vitani and Kovu are all Ziras, but only Nuka is the lovechild (if you can call it that) of Scar and Zira.
About Scar and Zira:
Do I believe Scar loved Zira? No, I do not.
Do I believe Zira loved Scar? In some kind of twisted way, yeah.. Though I do feel it was more admiration and obsession rather than love, I don`t feel like she had a "crush" on him, more that she looked up to him as her king and leader, and that she admired him and wanted to be by his side because that would gain her a high position in the pride. I think Zira wanted power and control more than anything else, and that was more important to her than love.