Scar wasn't Kovu's father. Nuka zei "Scar wasn't even his father, just took him." and Kovu zei this when he was with Kiara under the stars: "Do u think Scar's up there?...He wasn't my father but he's still part of me.." I think Zira and Scar had Nuka but because he was so dumb he couldn't be king so Zira had Vitani and Kovu with a different lion. Scar saw that Kovu is good to follow his pawprints and become king so he accepted Kovu as his own son. Nothing more.
I realize people have already answered - and they are absolutely right - but I heard about something else u might find interesting. It's a small selection of information a fan collected prior to the sequel's release:
I'm afraid I don't know how accurate this is because I didn't see the original article, and it sounds like the magazine got certain bits of information very wrong anyway! But it seems like Kovu's character was originally going to be Scar's son. At some point the writers must have realized, "wait, she can't marry her dad's cousin!" and changed this (along with other details of the storyline). Though like I said, considering they wrote that Simba and Scar were brothers, I don't know how trustworthy the rest of the artikel is. Thought u might find it interesting though!