Why change their mind? You can also let them change your mind. Just because you disagree doesn't mean you're right and they're wrong. For these big ethical and moral issues, people are often completely certain their view is right but they can't explain why. The reason is probably because their society tells them they'll be a bad person (at risk of being shunned!) for thinking otherwise. Strong social punishment for holding a belief isn't a good reason that belief is wrong.