On the question of whether we supposed to be monogamous, I figure it's safer these days to keep it that way. However, I'm with a Pyro here, if u really wanna still chill with the 3rd party even after the euphoria has died then u may have a real problem coz it means u feeling them on that deep level.
Anyway, methinks the concept of marriage makes no sense. Like RR pointed out earlier, shit changes, people change, life throws shit at u and sometimes it's too much for people to handle. We should be able to handle Slight behavioural changes, but people can make complete 360s and nobody signs up for that.
I feel that as humans we are capable of monogamy but that some people are forced to sacrifice much more than others in order to sustain monogamous relationships.
Typing on my phone so please excuse any grammatical or other mistakes