Gay marriage vs. non-Christian marriage?

Some Christians say that gay marriage is wrong or immoral, because the Bible tells us homosexuality is a sin. Others add that marriage is a holy institution from God, or that marriage is a religious concept. Are there any Christians out there who are against atheists and other non-Christians marrying,... show more Some Christians say that gay marriage is wrong or immoral, because the Bible tells us homosexuality is a sin.

Others add that marriage is a holy institution from God, or that marriage is a religious concept.

Are there any Christians out there who are against atheists and other non-Christians marrying, then?

Being an atheist is obviously a sin, one of the most blasphemous ones! Instead of just disobeying one of God's rules, we atheists say he doesn't exist altogether.

And if marriage is an indeed a God-given institution, then why on earth should atheists be allowed to marry?

If I still lived in America, it would be perfectly okay with Christians for me to marry a man, but not for me to marry my girlfriend because the latter somehow is against the rules of that 'Christian country'. Even though, either way, I still don't believe in God and am therefore sinning all the time.

How does this make sense? Any Christians here who are opposed to non-Christian marriages too?
Update: Four sensible answers in a row? Where are all the fundies today?
Update 2: Oh, there they are. It's about time, guys!
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