Why does every normie take unfaithfulness and cuckoldry as a normal, given things in their relationships?

I keep seeing normies say stuff like "if she fantasizes about other guys that is fine with me. I fantasize about other girls". Seriously? This just shows that you merely settled for your partner and you don't actually love him or even think he's good enough. If the two parties truly loved one another or even just found each other adequate, why would they fantasize about fucking strangers? I mean, that just negates the point of being in a relationship. Did you get together just to ease your materialistic burdens such as paying bills? [PRIOR NOTE: This is just an example. Look it up if you don't know the definition of the word. I don't really fantasize about this] If I made posts on how I fantasize about beating females until they're black and blue and making them my slaves until they starve to death, I'd be called a psychopath and you'd tell me to "get help", even though I wouldn't be acting on them. What is different?