If you were honestly concerned about over-population and unwanted pregnancies, you'd commit suicide. Not only would you reduce the population, you'd ensure that you could never contribute to the problem of unwanted pregnancies.
It disgusts me some think the lives of innocent babies should be 'sacrificed' for their cause, but they won't sacrifice their own.
You have to exercise some judgement. A few cells are not yet an “innocent baby”. As for the rest of it, I think the best way to reply to someone of your mentality is to point out that human beings - generally - are pretty disgusting creatures.