That's not true, affirmative action is when you give one applicant preference over another because of race,sex,etc. Changing the admission standards does no such thing. To put it another way, let's say the legislature decided to raise the minimums instead; and it resulted in a less diverse body. By your logic, that's discrimination. It's not discrimination unless the minimums were raised with the goal of having a less diverse student body.
In the case of the fire dept, the minimum standards were lowered with the stated purpose to facilitate "promotion of certain ranks in the Fire Department," particularly women.