I think it's more complicated than you're making it out to be, Judith. To say "because they can" is no more informative than to say "because they're cheaters."
Everybody "can." Temptation will present itself, sometimes in nearly irresistible form, sometimes at our weakest moments.
Some people can't face the prospect of having their options forever curtailed. Those people will cheat during the engagement or even during the honeymoon, just to prove to themselves that they still can if they want to.
Many people may hardly even notice all the opportunities and never once feel tempted until the marriage is near failing for other reasons and the loneliness is unbearable.
Some people never pass up any opportunity, as long as they're sure they won't get caught.
And some just won't, no matter what.
Your article is interesting and written well, but you're oversimplifying a complex phenomenon in human behavior.