Section 18-1-704, C.R.S. 1999, provides the affirmative defense of self-defense. It states in relevant part: a person is justified in using physical force upon another person in order to defend himself . . . from what he reasonably believes to be the use or imminent use of unlawful physical force by that other person. An affirmative defense is a defense that admits conduct leading to the act charged but seeks to justify, excuse, or mitigate that conduct. People v. Huckleberry, 768 P.2d 1235 (Colo. 1989); People v. Reed, 932 P.2d 842 (Colo. App. 1996). The United States and Colorado Constitutions guarantee equal protection of the laws. When two criminal statutes prescribe different penalties for identical conduct, a defendant convicted and sentenced under the harsher of the two is denied equal protection of the laws. Statutory classifications of crimes must be based on differences that are real in fact and reasonably related to the purposes of the legislative enactments. The General Assembly, however, may establish harsher penalties for acts that it believes have greater consequences, even if the differences are only a matter of degree. People v. Oliver, 745 P.2d 222 (Colo. 1987); People v. Mozee, 723 P.2d 117 (Colo. 1986).