You don't have to remain friends with them. Especially if you have told them (I assume you have) that your choices are yours and you have no interest in changing them or discussing them.
I have many friends who are Christians that I've known for a decade if not longer and this issue doesn't come up. They believe what they believe, I believe what I believe, we're all aware of it and it doesn't affect our friendships.
Surround yourself with people who accept you for you.