Im an American marrying a British girl do you lose your citizenship?

No you're an American. It'll be a pain in the backside to get her here though. Unless the US has an exception for the UK. Its usually easier to get her here officially to marry her first and then marry her, than to marry her and try to get her here after you're married. From everything I've read on it. I think its dumb that we can marry who we want without government approval, but people can jump the fence and live here tax free.