The end does not justify the means; acting for a good purpose does not make the act good.
Thankfully, another great Moral Theologian, Janet Smith, has summed (or is that summa-ed) everything up.
Widespread Use of Contraception Will Lead to...
1. Lowering of Morality
2. Disregard of women's well-being
3. Coercive use of contraceptive method by governments
4. Treating our bodies like machines
5. Increase in divorce
6. Less bonding and growth in virtue between married couples
7. Contraception facilitates adultery
8. Sexual involvement before marriage
10. Increase in unwanted pregnancies and abortion.
(as predicted by Pope Paul VI in Humanae Vitae)