You might have had a point if expecting fidelity was crazy.
But it isn't.
It's hardwired in most people that being cheated on hurts - which makes "fidelity by default" a pretty logical expectation. And if most people expect it, it's a bit weird to say that "it's crazy". Or the vast majority of people is crazy and you're the only sane man. I'll go with Ockham's razor here.
Anyway, cheating imply a relationship, and a relationship is BASED on two persons - and it means their opinions and expectations too. It seems pretty obvious, again, that taking into account logical expectation is, and should be, the norm, not the exception.
And even if the expectation ARE crazy... well, you KNOW they do exist, and as such it's logical to think that the other person has them by default, so...