you're trying to show how, via internal critique, the Bible commands other things which aren't followed, which therefore negates the things which are accepted? But, purely logically speaking, this proves only hypocrisy, not that gay marriage ought to be accepted. It simply proves by example that a wife not a virgin should be executed. I doubt you're arguing for that, so let's go to the next step.
You're most likely arguing that we ought to reject a literal reading of the Bible (anti-gay marriage) with a literal reading of the Bible (pro-stoning of non-virgin brides), except that (and here's the kicker) most people have always already rejected your literal reading of the Bible. You can see, I hope, how this is self-defeating. They hope to show that you're only being selective about your biblical passage, which is your very critique of their position. The ball really isn't moved forward here, except to point out hypocrisy. See point #1 above. There are 613 commandments in the Hebrew Bible. By your logic, one would only need to violate one to invalidate all of them. Seems odd. Perhaps we could apply the same standards to all politics. If that were the case, however, we'd all be voting Green or Libertarian.
Perhaps when you're arguing against the way in which the Bible is literally or selectively interpreted, if this is what you're doing, then you may want to to show which passages one ought to select in order to be true to the document. But if one does this, then one is also being selective. And so . . .
Other options? I'd really like to figure out how this approach is effective in demonstrating that anti-gay marriage opponents ought to either 1) stone non-virgin wives; 2) stop being hypocrites (but even this isn't an argument for gay marriage; it's an argument for stoning non-virgin wives, so stop being such a misogynist 
; 3) become Greens or Libertarians.
That is all.
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