Why do gay people want to get married in a church whereas they have civil union?

I have nothing against gay people and I believe in civil union which confers the same rights and privileges as marriage. But I don’t understand why some of them want to get married in a church. Marriage is a religious institution and the sacred union between a man and a woman. Allowing them to marry in a church is disrespectful to most Christians. We should have the same rights as citizen but our religion which has nothing to do with the state should be respected.