I absolutely agree that every time the effort should be put forth to be civil, but at the point of which they continue in such a way that is divisive what do we do?
And you're right, I'd never thought about the fact that I could have "planted a seed." Change often does take time.
What you can. Yet don't drop to their level. Act like the person you'd hope other people would be like. Don't sully yourself due to their intractable nature.
I think in any controversial subject or where you have someone acting in defense of their beliefs it almost always takes time. The cases where it does not are actually pretty rare.