That's retarded. The distinction between the two words "gender" and "sex" has nothing to do with the fact that two genders are a reality. They're just trying to divert your attention to a semantic issue in the hope that you'll lose track of what you were trying to say.
Just say: there are two sexes and two genders. The fact that sex and gender are different concepts doesn't change that they are intimately related. You say that gender is a construct: prove it to me.
Anyways, in some sense "gender" is not real, since all will be united before God, and there is no distinction between man and woman when they are each being judged before God, so definitely it makes sense to say gender is "not real". But at that level, race, occupation, personality, ego, etc also disappears. The same people who obsess about gender seem to care an awful lot about other concepts that aren't real, like social class (not real) and ethnic identity (not real).