I think people compare the TLK lions to real life lions' behaviour way too much in their peminat theories. I mean, seeing to the fact that the TLK lions are clearly human in essence and value system - including knowing and using the concept of even arranged marriage. So I really don't think real life lion males' habit of mating with any and all lionesses that would let him, would apply here on any level. And if it did, then it wouldn't even be cheating as it would be a natural system in their society.
But it wouldn't even fit their world. I mean, if they had that habit, the king would've been used to sleeping around from the beginning of time and so even Mufasa would've had that value system, which would lead him to have cubs with lots of lionesses and then Scar would've had way too many cubs to do in. Plus, Mufasa was clearly a first-time father. (That's my opinion based on his choices and lack of noticing a certain attitude from Simba, in the scene where he dealt with Simba's disobidience with the whole gajah Graveyard matter. But I digress.)
And yes, humans may cheat on their partners. But does Simba come across as someone who would? In my opinion, hell no.
First of all, mentally/at hati, tengah-tengah he was still very much a little boy - he never had the chance to grow up when living with Timon & Pumbaa. Yet he had to play the role of a grown-up, all the sudden. And even ended up having a child only months after reclaiming his throne. That's hell of a lot scary responsibility for someone who is at heart/mentally still lebih of a 10-year old kid. I very much doubt he had sex in his mind all that much at all, not to mention doing it with anyone else but the person he fell in Cinta with.
And he was his father's son and tried to be as much like Mufasa as he could. It's unlikely he ever saw Mufasa cheating on Sarabi. Mufasa was way too high moral and respectful for that, so SImba would've had a good role model.
And, if he did mate with anyone else but Nala, Kiara wouldn't be his only child which she obviously is. As human as these guys are, they do not and cannot have birth control.
And, as said, he seemed to have his father's value systen, so if he wanted to sleep around, he wouldn't have stayed married to Nala but devorced and erased atau changed whatever laws there were about marriage. But, as we see in TLK2: Simba's Pride, he very much respected his father's laws instead of going around changing them to suit his desires.