Well, there's no "just" about it. A social construct is a social construct, and it's as valid in its way as any other concept. The only question about a social construct worth asking is whether it's a valid or useful social construct, and whether contradictory social constructs might be more valid or useful.
I expect nonsense like "just a social construct" from left-wing idiots, but it depresses me to hear it from libertarians, who are supposed to be logical. I dealt with that annoyance HERE. And over at http://therightstuff.biz/ they have a longer treatment of the subject: