Actually, I think nobody should be forced to "prove" anything in regards to this issue. Both sides are laden with opinion and personal ideology. Asking someone to prove the merit of their personal beliefs is arrogant and inflammatory.
Abortion wasn't legalized to encourage people to kill their unborn children. It was legalized because the government finally decided that the wisdom or evil of that course of action boiled down to an individual's personal code of ethics. Legalized abortion was an admirable act of giving americans more personal freedom, as well as providing a safe option for the truly desperate. However, you can't make everyone happy. Some people believe that freedom should conform to what
they think is right, but is that really freedom or just fascism disguised as compassion?