Every now and then, researchers observe a chimpanzee harem, and note that the females often try to sneak away from the dominant male, to have sex with males that hang around the periphery of the harem.
They then say that because chimpanzees cheat, that it must be natural to cheat on one’s spouse.
However, they always neglect the other side of the equation. If a dominant male catches one of his females cheating with another male, he is likely to brutally kill both of them.
So, if cheating is natural, so is murdering your cheating spouse and their lover if you catch them. Just like in Texas.
And for the "no morality without religion" crowd, there is Leviticus 20:10, which says the same thing.