I am sure you mean "science teaches that there *are* only two sexes." But I am pretty sure that you are wrong about today's science. Does lgbqt not ring a little bell for you, Sir John? 

Moreover, "gender" is a grammatical term (look at French and Italian and German) though it has migrated through loose usage and prudery into many spheres of life. Strictly speaking, which nobody ever does anymore, language has gender; human beings have sex.

Think, Sir John, think, before you write.