Not sexist, stupid.
So-called "cocktail casual" is whatever the twats that drink cocktails decide is fashionable this year. Those of us in the RealWorld point and laugh at idiots who spend over half their income chasing a clothing fad that never stops moving.
Neither my wife nor I pay any attention to such stupidity when invited to attend functions. Somehow, we have never been asked to leave due to our incorrect attire ... but occasionally we do manage to stick out. Which invariably pisses off the idiot who set the supposed dress code. I wonder why?
 Exceptions: weddings and funerals. The first is about the bride, not the guests. The second is honoring the deceased.
 If you want to call me sexist for that comment, I'll cop to it ... HOWEVER, in our Western society I have never attended a wedding where the Bride(s) was/were NOT the most important person(s) in the room.