"Defenders of sex-based government action must demonstrate an exceedingly persuasive justification for that action to make that demonstration. The defender of a gender line must show at least that the talents classification served important governmental objective and that any discriminatory mean employed is substantively related to the achievement of those objectives. The heightened review standard applicable to sex-based classification does not make a proscribed classification but it does mark as presumptively invalid incompatible with equal protection a law or official policy that denies to women simply because they are women equal opportunity to aspire, achieve, participate in, and contribute to society based upon what they can do. Under this exacting standard reliance on overbroad generalization typically male or typically female tendency estimates about the way most women or most men are will not suffice to deny opportunity to women whose talent and capacity place them outside the average description. As this Court said in Mississippi University for women against Hogan some 14 years ago state actors may not close entrance gates based on fixed notions concerning their roles and abilities of males and females."