Just saying this from an anthropological standpoint, humans have very little different sexual differences and that among primates is generally the sign of monogamous preferences. That being said, why are people not bringing up the idea of having open relationships? I wouldn't cheat on someone. Personally, I think people who cheat generally do this because of problems in the relationship. I address those before finding a girl or a boy to bang me.
Back to open relationships, if people are as 'bored with monogamy' as people here claim, then I expect to see open relationships flourish (now that the stigma towards them is lessening) within the decade.