@hawkeye10,
Your theoretical sentiment makes sense if you don't take into the account of the substance of the actual costumes.
Boys tend to choose scary costumes of monsters or if they choose to pick a adult mentor or hero costume (fire fighter, police, military, etc...) the costumes are straight and narrow and without any hint of sexuality.
But for some reason, adult women
have (choose) to dress up in some highly exaggerated and oversexualized versions of whatever they are trying to mimic in costume form no matter if it's a real life character or fictional fantasy character.
Eg: sexy police officer (where a real police woman wouldn't be wearing daisy duke shorts as her uniform); sexy fire fighter (where a real female fire fighter would wear a shirt and bulky fire resistant jacket and not merely a glorified bra); a sexy witch; a sexy zombie (oxymoronic?); a sexy construction worker; etc.... All these costumes being a variation of a very short skirt or pair of shorts and a top that would reveal a lot of cleavage.
I think the point of the professor is that girls are pushed to wear quasirevealing genie costumes; princess costumes; etc....