At Letters of Note we find this letter from Ronald Reagan to his son Michael in the days before Michael's wedding. Basically, it's about the true value of remaining faithful and in it he provides the best definition of being a "man" that I've seen:

Sure, there will be moments when you will see someone or think back to an earlier time and you will be challenged to see if you can still make the grade, but let me tell you how really great is the challenge of proving your masculinity and charm with one woman for the rest of your life. Any man can find a twerp here and there who will go along with cheating, and it doesn't take all that much manhood. It does take quite a man to remain attractive and to be loved by a woman who has heard him snore, seen him unshaven, tended him while he was sick and washed his dirty underwear. Do that and keep her still feeling a warm glow and you will know some very beautiful music.