Reasons to Believe, or Reason vs. Belief?

My friend the Friendly Atheist has posted a (liberal) pastor's critique of presidential candidate John Edwards, who says that his opposition to gay marriage is "influenced" by his Southern Baptist background. The pastor says in response:

Sen. Edwards said his opposition to gay marriage has [been] influenced by his Southern Baptist background. Most Americans agree it was wrong and unconstitutional to use religion to justify slavery, segregation and denying women the right to vote. So why is it still acceptable to use religion to deny gay Americans their full and equal rights?

To which I ask the following questions. This is just off the top of my head, and doubtless there are still more (and better) questions that could be asked in response to this pastor's sloppy thinking:

1. So are we saying that opposition to gay marriage is only OK if it's NOT motivated by religious reasons?
2. Why should religion be excluded as a reason for having a political view? What makes it less valid than any other reason? Says who?
3. If we're going to exlude religious conviction from the public square, then what if the reason given for a political position is 30% religious and 70% "secular" - is that OK? If not, then what if it's 5% religious and 95% secular? What if we disagree on how religiously motivated my position is? Then who gets to be umpire?
4. What good is a religion that doesn't have any effect ("influence") on how I think about life issues?

And finally, if it was wrong for some to use religion to justify slavery, was it also wrong for Wilberforce to use "religion" as a reason to fight for its end?