While being angry can lead to angry words, it's not productive. So I just wrote him a much more positive note; it's copied below.
I hope you'll find yourself motivated to do the same; nothing good ever came of negativity. However, we can help each other, even - and especially - when we disagree.
Dear Governor Douglas -
You've made a mistake.
It's OK; we all do - maybe you got some bad advice; it happens.
However, as with most mistakes, it's correctable - and, here's the great part: When you rise to the occasion by doing the right thing, you end up in an *improved* position - even relative to *before* the mistake. A beautiful thing "enlightened self-interest" is.
So, please *do* the right thing: Do not veto the gay-marriage legislation.
You might refer to the "will of the people" - which I'm sure is making itself extremely clear right now. You might wait until the bill is presented to you, agonize over it a bit, and then graciously and humbly admit that allowing it is, after all, the right thing to do - and it will correctly be viewed as courageous. Obstruction never is.
It is not only the right thing to do, it is also the best course politically.
Thanks and good luck,