Just because nothing can be proven nor disproven does not mean there can not be facts involved. The existence of God is not an opinion, because it is on a factual basis. God doesn't exist for some people. We are just too ignorant to decide what is fact. I have already shown how God would be improbable with logical reasoning, which is definitely not a matter of opinion.
And even opinions can be argued about. If two people disagreed about which of them was the best person in the world, they would argue.
Insulting a stupid action does not generally start massive debates that swallow up topics and end almost exactly as they started (save for a few angry people) after going on for hours. In real life, I hope you don't go up to some random Buddhist and insult what they believe in. The only reason people seem to do it here is because there's no consequence for them for doing so.
I would debate with people, although I would indeed restrain my language. Fact is, on this forum people don't have to be stopped on the street. They can choose to ignore people and they can respond whenever they want.