Why does ethics have to equal religion?

I've been keeping a close eye on the lead up to the US elections. I'm not sure whether I've become more politically aware, or whether I've just been following more US blogs and podcasts over the last few months. One story that I spotted today was on the good old BBC, however. It was a debate between McCain and Obama. Nothing unusual so far. The odd thing in this case was that it was billed as a 'religious forum', hosted by the pastor of a mega church (that phrase makes me shudder) called Rick Warren.

The reports I've read so far don't really spell out why the questions asked of the candidates were particularly religious and why they couldn't have equally been asked in a secular context on the ethical and moral beliefs of the two men. Abortion, same-sex marriage, teen pregnancies and, more personally, "What was your greatest moral failure?", "Who are the three wisest people in your life?", "Why do you want to be president?".

Why do questions like this need to be asked in a religious setting? If I wanted to vote for someone, I would want to know how he came to hold his views on a particular subject. If somebody is against same-sex marriage or abortion in any situation, then I'd expect to hear reasonable argument about why that is a sensible position to hold. Just saying "That's my position because my religion says so" doesn't give me a lot of confidence in someone who has to make important decisions based on many sources of evidence and opinion.

To pick out one of these "moral" issues, I find the obsession with same-sex marriage particularly annoying. Apart from the obvious point that it's nothing to do with anyone else, the argument often seems to over a matter of semantics. Obama was quoted as saying that marriage should only be a "union between a man and a woman", but he supported same-sex civil unions. If there is no legal difference between a marriage and a civil union, then is the argument just over m-word? Some even argue that the only true marriage is a religious one. Of course, what they mean by this is a Christian marriage ceremony, pushing away other religions as well as secularists.

I once heard someone from one of the humanist organizations suggesting that we could leave the word 'marriage' as the description for the Christian ceremony and everyone still goes has to go through a civil process to formalize their union. At first I thought this was a good idea. Of course, everyone would still refer to the civil unions as 'marriages' and 'weddings', whatever the formal designation. The other reason it would be a bad idea is that marriage didn't ever start off with religious connotations. Marriages were going on long before it became a Christian ceremony in the Middle Ages. The Romans even had legal same-sex marriages!

It would be interesting if any politican who made a statement on a moral matter actually explained why they held that view rather than just refering to the standard line of the party they belong to or the religion they happened to be born into.