Can a relationship with Islamic/non-Islamic people work out?

I am going to be honest with you, my sister married a Muslim man so I know. Muslim men can marry a non Muslim but the children have to be brought up Muslim. Muslim woman can only marry Muslim men. Some Muslim men do have relationships ' outside marriage' but that's as far as it goes. It's a fling not relationships, yes he does sound very nice but he is obviously VERY into his religions, with respect I cannot see this as going far as a relationship.