The trend in our evolution is towards monogamy, but to make a long story short: we're not there yet. If monogamy were truly 'natural' for us, in the sense that it is for certain other species, it would take no effort at all. There would be no temptation to cheat.
I think its the other way around,people want to be monogamous or appear to be for some reason,but in the practical way they cant be,I could be wrong,but I would guess people are getting much more promiscuous and cheating more.