So we all grew up watching The Lion King. Simba's dad (the King) is killed, young Simba gets exiled, meets some cool friends and then his childhood friend Nala comes to find him, he returns, overthrows the evil new King and has his own family with Nala. The circle of life goes on.
EXCEPT.
In a pride of lions, there is just one male. He has several lionesses who he has cubs with. The male cubs, on reaching puberty, are exiled from the group and start their own prides.
So who's the girl cub, Nala, that Simba plays with? She's his sister.
And when she comes to find him as an adult, things get somewhat steamy and then Elton John sings that song.
This is why young male lions get sent away from the pride, to prevent this kind of behaviour!
And then the happy ending, where brother and sister shack up and have a lion cub all of their own.