Why does everyone think boundaries are good and that they work?

Somehow it seems that the christian idea of boundaries governs everything. Once married there are 'rules' to stifle natural urges and to dictate behavior in a given situation. Overlooking the fact that life expectancy has dramatically changed in the last 75 years, the modern world seems to take such rule bound scrutiny less and less well. Why can't relationships be based on how much people genuinely like each other and not on fascist demonstrations of love? Not necessarily 'open' marriages, but more forgiving ones that allow for natural impulses seems more human, doesn't it?