I see you have compared sex to race and sexism to racism. However, are you also willing to take that comparison one step further and agree that like race, sex is also a social construct based on appearances?
No, absolutely not, that is where the analogy falls down. Ethnicity is indeed a social construct, as are countries. But sex is a biological fact.
There are many different ethnicities, many overlap (however you choose to categorise them), but sex is a distinct biological category distinct. Also there is no spectrum. You are either intersex or you are not, this is a clear binary. The vast majority of people are not intersex, a miniscule proportion are, and all are female or male, there is no third sex.
You are correct that biology is complex (I would add, fascinating) but you mistaken to think that feminists or those stating the fact that there are two sexes don't already know this.