Reproductive organs exist and separate the behaviors and life cycles of the members of a species... whether or not human culture exists to assign them significance.

So if the argument is that sex characteristics, of which reproductive organs most certainly are one, have no meaning or power outside of that which society places on them, then it's obviously false. Because we see throughout the animal world that sexual characteristics drive and define behavior even in species with nothing we would recognize as a society.

There is significance to sex and sex organs/characteristics outside of what society places upon them. Because sex is how (many) species reproduce, and most individuals have a strong biological drive to reproduce (or at least to take part in the sex act which would normally risk reproduction). Whatever else society does or doesn't pile on top of this, this significance predates both it, and society itself.