[throwaway729]

>...would generally be considered impolite in real life

1. Neither would insisting that someone have a public debate with you. The entire situation was not at all like "real life".

2. I've been on both sides of "we should stop talking about this because we'll never agree and won't learn anything". I've never felt uncomfortable or rude on either side of that conversation.

[nikcheerla]

In real life, if you publicly proclaim controversial beliefs, people will probably publicly confront or question you about them. Most of them will not push a debate further if you say "I don't want to talk about this topic" or something similar. But if you say "We seem to disagree about some basic facts", it seems to imply you think they are making basic/obvious mistakes, but you aren't going even going to deign to explain those mistakes to them. In short, it could come off as condescending or rude, and it could easily be averted by being a bit more polite.