People misunderstand "social construct" to mean "whatever the fuck people say", but this is not the case. A social construct is the result of social function, when the people behave according to their free will and actualize their preferences, wants and needs.
For example, because we need to travel, we have built roads to connect communities. Roads are a social construct. Because we don't like to spend too much time traveling, we have invented cars. Cars are a social construct. Because fast moving cars sometimes result in deadly accidents, we have also created road fatalities. Car crashes are a social construct.
You can see how we can't exactly choose not to have car crashes unless we undo a whole lot of other premises and totally change how our society exists in the first place. Likewise, because we need to have babies to exist, and because we want a division of labor that utilizes the differing abilities and preferences of male and female individuals, we have constructed the genders of men and women. These constructs are a result of how our society works, not what people simply declare for themselves.
Ergo, when you go around claiming you're a woman even though you were born with XY chromosomes and a penis, or vice versa, what you're actually doing is rejecting the entire premise of the society around you. When you do that, there's no reason whatsoever for the society to take you seriously - you're just a raving lunatic.