Gay marriage vs. Straight non-religious marriage

My position on state recognition for gay marriage was (and remains) roughly thus:

• The State should not be involved in marriage at all, whether for straight couples, gay couples, or polygamous groups.
• Thus, I opposed it being made legal, as this involved the state claiming the right to define marriage, This is as opposed to it merely being legal. I now oppose it being de-legalised for the same reason.
• From a religious perspective, I do not personally see a marriage between two people of the same sex as being valid.
• However, freedom of contract implies that two people who wish to have a contract between them which does not affect anyone else should be allowed to have that contract. If they wish to call it marriage, then that's their choice.

Th third point there is probably the most controversial. I see the fundamental purpose of marriage as being an illustration of the relationship between God and His people. God is an essential part of a marriage. This leads to a question which I'd never considered or even though of before it was asked on me on Tuesday by a housemate:

Do I see a straight marriage between two non-Christians as valid?

Since I see God as a fundamental part of a marriage, my instinct is not to recognise such as marriage as valid. This has important implications. Since I also believe that sex outside of marriage is wrong, answering "No" implies that I should believe sex to always be wrong for any non-Christian, "married" or not. It would not require me to advocate banning non-Christians from getting married, as explained above, but it might well mean that people who do advocate a ban on gay marriages should also advocate a ban on non-religious marriages.

There are perhaps ways of escaping this. Perhaps there is another crucial difference. The most obvious attempt would be some kind of Natural Law argument - that non-Christian marriages still serve a natural purpose of bringing new children into the world. However, I find this unconvincing - perhaps new children are brought into the world, but if those children are not brought up to be Christians, then is this really fulfilling Natural Law?

Perhaps it is that a straight non-Christian marriage has the potential to become a Christian marriage if both partners pledge themselves to Jesus. This simply isn't the case with a gay marriage. However, why then not say that the marriage becomes valid only once the partners have both committed themselves to God, and was previously invalid?