Remember equality before the law? It applies to affirmative action. "Affirmative action refers to policies that take factors including "race, color, religion, sex or national origin"[1] into consideration in order to benefit an underrepresented group, usually as a means to counter the effects of a history of discrimination" (Wikipedia).
As I understand it, there are two kinds of affirmative action: affirmative action in government actions about government employees/admission to public educational institutions/etc. and affirmative action taken by private institutions, companies, etc. I want to consider both.
First, a point that needs to be made is that affirmative action is bias toward particular groups based on race, color, etc. (things we generally consider characteristics that should not be held *against* someone in the hiring process, even though many people are okay with 'valuing diversity' and giving the job to someone in an 'underrepresented group'). My point is that equality before the laws implies equality
NOW. Although those underrepresented may be disadvantaged compared to their situation had their parents not been discriminated against, the inequality was not directly against them. Even if affirmative action for an individual who was directly discriminated against could be justified, giving an advantage based on race, religion, etc. cannot be neither politically justified for governments nor morally justified for private individuals (except for positions like Black Americans Group President or Methodist minister).
Second, one cannot give preference to one group of people without disadvantaging others. It seems nicer to talk about giving preference to the children of those who have been discriminated against in the past--and whose discrimination has adversely affected the children's situations (not all discrimination does this in every case)--than to disadvantage someone who has worked hard whose parents have not been historically discriminated against for being White, Protestant, or heterosexual, doesn't it? Yet, it is intuitive that giving preference to one group discriminates against another.
Government Affirmative Action
Some people think equality before the law meshes with affirmative action. The government is trying to bring about equality between the various social groups, right? However, I think some people who realize that preferring one social group disadvantages another would see that equality before the law cannot coincide with a purposeful disadvantaging of any social group, especially in the name of equality. Therefore, affirmative action is improper for a government of the people.
Private Affirmative Action
Affirmative action by private institutions and individual is something quite different. Private institutions are not institutions 'of the people'. We do not have the right to be equal before Hannaford, McDonald's, Steckle and Smith Law Firm, or Boston Medical Center (assuming these are privately run). They can choose whom they desire, valuing to the extent they wish or not valuing the social groups from which they applicants and employees hail. Whether it is morally right for private individuals to value 'diversity' or not is not my concern here. A company inflicts no harm on individuals by not hiring them, not giving them a 401k, or not giving them vacation time. If they don't want the job with the pay and benefits, they don't have to do it.