The mere fact that something is a social construct does not mean that the thing has no essential features. Construction of all kinds is constrained by rules. For instance, the construction of a stable building requires taking into account the law of gravity. You can point at any given building and say “it could have been otherwise,” but you cannot say that “all ways of erecting a building in this spot are equally correct,” as many of the ways of building the structure will result in said structure collapsing.
Likewise, you can point to our understanding of gender and say “it could have been otherwise,” but you cannot say that “all ways of understanding gender are equally valid.” A good account of gender cannot ignore human sexual dimorphism or the reproductive function of sex, which is why a binary understanding of gender has been so common across cultures. It has prevailed because it has been useful.
Language is another example of something socially constructed that is bound by rules. Just as an understanding of gender which does not meaningfully account for sexual dimorphism is unlikely to be useful, so is a language without some sort of grammar unlikely to be useful. There are differences between the grammars of different languages, but they all have grammars, because the grammar is necessary for the language to serve its function, i.e., facilitating communication.
The fact that buildings, gender roles, and language are made by humans does not imply that the construction of buildings, gender roles, and language is constrained by nothing except the boundaries of our creativity. There are rules that we can’t just wish away unless we plan on doing away with the things themselves. The constraint of being useful for communication is constitutive for language, for example. Without such a constraint, there would be no language. And insofar as the things themselves are useful, as buildings, gender roles, and language certainly are, we should not discard the constraints from which those things derive their being.