Responding to some of the recent posts that I've agreed with: Again guys, it's this insane but common philosophy, that things like "language" and "math" is unnatural but things like eating are natural. Ultimately if it requires thinking (reason), we call it "unnatural." So it's unnatural for us to make cities and such (which makes me wonder how, if that were so, we're the only animals living in blatant defiance of our own natures––I don't think that we do defy it nearly so much as we 'think').
I think it's entirely wrong to suppose, that reason is unnatural, or that our "social constructs," which are our rational constructs, don't have objective referents existent in nature––they clearly do, and human reason itself is existent in nature and functions within the context of nature. Even living in brick houses and shittalking on NeoGaf strikes me as being equally "natural" for human animals as flying is for birds. How could it possibly be otherwise when we're here doing it?
I'm going off-topic but this (simplistic) duality is extremely prevalent and has always rankled my feathers (no pun intended). To answer the actual thread question, I'd say monogamy is both natural ("real") and rational ("social construct"), and I don't think we'd even conceive the concept of monogamy if it had no objective referent. Step 1, me thinks, would be taking a hammer and breaking the presupposition that creates the question ("Is it real or is it 'just' a concept?") in the first place.
These dualities between subject and object, as someone just said (which was well said!), are holdovers of some bad Enlightenment philosophy and, I pray, will one day be put to death. Schopenhauer already ranted against this in 1830s but no one has listened!