Not everybody is that demanding of their partner though.
Hmmm most people are. That is why adultery is such a big problem among monogamous cultures that there are secular and religious laws against it.
Monogomay might be flawed, but i think its better than all the other alternatives.
Which other alternatives? Is culturally enforcing monogamy on people who are not inclined towards it, better for them? With divorce rates at over 50% and adultery epidemic, perhaps it's time to reconsider if compulsory monogamy is really such a better option for most people

Sure we mess around in certain phases of our lives, but on the long run I do want someone who is loyal and i can rely on and vice versa.
So sex is the only kind of loyalty?
Sex is a hangup for most of us because of centuries of religious dogma. We make exceptions for sex in ways we don't for other things.
E.g. most married couples have friends, some very close friends, who they tell things to that they don't tell their spouses. Is that cheating?
What about spending money behind your spouse's back or on something they didn't want you to spend money on?
My point is just that, we can think of sex as the most important thing in a relationship, which is "owned" by your spouse, at the exclusion of all others, that is monogamy.
Or, you can see each other as individuals, who come together and share a life, love, happiness and yet do not "own" each other's sex.
Centuries of religious and patriarchal dogmas have made this seem like such a foreign idea, when there is anthropological evidence that this is how our closest primate relatives live, as well as hunter-gatherer tribes lived for millennia, until the advent of agriculture 6-10 thousand years ago, which also was the advent of patriarchy, the nation-state and organized religions.