Cheating the partner is sexual immorality. In the Animal world their is no feeling of belonging. Only the dominant male gets every or the best of the female species.
In the human world also best looks and good status gets you the best of the girls.
similiarity ends there. in the human world the partners are expected to be loyal to one another. if not the price has to be paid.
the price is paid by the partners themselves or their children.
in the animal world the kids or cubs grow up fast and in a few months to a few years they stand on their own feet. they can fend for themselves.
in the human world the children are dependent on their parents for a term of 20 years.
that is the reason why the partners or the parents should be faithful to each other.
no man will rear the child of the other as he would his own child. no mother will rear the child of the other as her own. the exceptions could be their. but this is the universal truth.
so if the parents cheat on one another they pay for that themselves and also their children end up paying for that.
being born as a human being if they cheat they create a jungle world and live in the jungle law for themselves.
so being faithful to the partner is sexual morality