But getting upset over someone's opinion, no matter how stupid it is, for a lack of a more PC word, is retarded.
Absolutely. But that's not exclusive of the Internet.

Who cares if they are acting like their opinion is a fact and that anything you say is wrong?
I would actually hope they would act like their opinion is a fact. But that's another discussion entirely.

Also, one thing you must understand about arguing on the Internet: 99.5% of the time, no one's opinion is going to change because of you.
Again, not just the 'net.