Interestingly enough, you are not the first person to ask that question.
The short answer is “In the same way and manner that everything is a social construct”. Which is not that satisfying an answer, *and* it assumes you aren’t operating under a veil of ignorance about what social constructs are (which the overwhelming majority of people are doing).
The long answer I have already given, and you can read it here: http://tonidorsayquotes.tumblr.com/post/89900712845/this-is-something-ive-been-wondering-since-i-saw-it-on
If, at the end of it, you do not realize three things, then you need to get some strong foundation stuff under you first in terms of ideas, history, and concepts.
Those three things are:
1 - All social constructs are real.
2 - Those who think sex is not a social construct do not understand what they are talking about.
3 - It is an outright lie to say that sex is not a social construct while saying that *anything else* is a social construct.
(shakes head) You come into MY house, you tell me MY favorite characters are heterosexual…