I spent some of this weekend reading What Do Women Want? It is a book about female desire examining both the classic beliefs we see reflected in popular culture and the actual science that is being done to understand the topic. It is a fascinating book, fairly short, and takes an interesting tack; that is, that monogamy kills desire and that our attempts to 'cure' the boredom and lack of desire that comes from long term monogamy through pharmaceutical aids or therapy are feeble at best. I should note that much of what I say here is going to be heteronormative both because that is where the book focuses and because I can actually talk about it with some degree of personal experience.
There is an assumption made that what women need in relationships to kindle desire is intimacy, trust, cuddling, someone who takes an equal part in chores and childrearing, etc. While all of these are good things I don't think that trying to fix a lack of desire in a long term relationship this way is actually useful. The raw desire for someone else that occurs in a young relationship is powerful enough to demolish pretty near any kind of barrier that may arise and we know from endless examples that women don't suddenly go back there because their man decides to do laundry or snuggle.
The book doesn't present any sort of solution. It outlines all kinds of problems with sexual desire in long term monogamy but never tries to tell the reader what exactly might be done about this. It seems to me that the author simply recognizes that opening things up to a discussion of polyamory or open relationships is too much to just tack on. Trying to cover that would be too much for a book that tries to focus on a smaller, more narrow topic but it does end up feeling like What Do Women Want? simply ends up concluding that everything sucks and you can either have endless short term relationships or just accept the end of lust and desire.
That isn't to say that monogamy is bad or that it will automatically leave people unhappy; neither is true. It is certainly true though that the great majority of ostensibly monogamous relationships have cheating as people chase the rush they once felt. It is also true that an awful lot of people desperately wish they could have sex the way they did at first and find no solutions. I think this illustrates so clearly why we would all be better off if our culture presented monogamy as an option for those who find it works best for them instead of as the only morally acceptable choice.