It's not a sin. The Gurus taught us that all people are equal in the eyes of God. and all people have their own way of reaching God. Just because we are Sikh does not give us "exclusive" access to achiving salvation with God. Everyone has equal right to reach God and we cannot judge them on their way of viewing God. We are all equal in the eyes of God. Children will find their own way to God just like everyone else. There is no right religion, there is no right way to view God. In the end we are all the same. We are all equal. We can marry whoever we want in God's eyes.