And, again, I'm simply saying that with so much of this discussion having the qualifier of defining infidelity as being where all of the parties haven't consented, she may have assumed that was understood and left the qualifier off. And, having that doubt, I'd prefer to ask if it was her intent to call consenting parties shits or if she really meant just those who go behind their partner's backs.
If she meant that someone whose partner says, "sure, honey, go bang so-and-so", then I have issue with the position -- if she meant just those who sneak, lie and betray, then I support it; because that does harm, not only to the betrayed partner, but often to every future relationship that partner may have.
There is "no reason" to do something that so betrays the trust of someone you've made a pledge to.