who believes that some people have an honest, natural, innate trait that makes them desire an exclusive sexual relationship more than a relationship equal to the aformentioned except that they're not bound to sexual exclusivity?

IOW, without explaining how you know, let's say that you know that your SO is exclusive to you, and you know that your SO is totally cool with you not being exclusive to him/her. how many of you would still remain exclusive because you honestly would not want to have sex with anybody other than your SO?