I'm not judging. People may live however they want. Institution of marriage is one of the choices, that is established in the society and carries with it some particular set of features. In particular, joining in marriage is somewhat public, even if it's much less public than it used to be. Also marriage comes with explicit (even if not legally binding) expectation you don't cheat. If you don't like those aspects, don't marry; no one is telling you have to, open relationships are fine too.