I've always opposed abortion on non-religious grounds. In addition to causing the often unnecessary destruction of a life or a potential life, abortion can encourage sexual irresponsibility. If you fail to take advantage of all the readily available methods of birth control such as condoms, the pill, etc., you shouldn't be able to simply avoid the consequences of your irresponsibility by terminating your pregnancy. If you have sex, pregnancy may result. You knew that going into it. Live with the consequences instead of destroying a potential life.
Abortion should obviously be an option in cases of rape or serious threats to the mother's health.