It seems that about once a month, I have a variation of the exchange above. My guess is that it's a "generational thing" (does this mean I am finally old enough to be alienated from the younger generation?!) resulting from 1990s political correctness, which taught us that everyone's opinion is unique and valuable. That's not true, of course, but the consequence seems to be that "it's just my opinion" is the new excuse for saying something really stupid. Since opinions are all, well, just a matter of opinion, saying "it's just my opinion" tries to shelter your words by simultaneously discrediting them and leaving oneself open to counter-attack, as illustrated above. Saying "it's just my opinion" is a sign of weakness; you want to avoid conflict but you're not smart enough to realize it so your actions lack resolve.
If you don't enjoy being dished on in public, you probably shouldn't dish on others who might retaliate in kind and who might not back away from conflict. I'm not here to offer strategic advice, but a better strategy for handling a response of "hey! that was rude!" is apology or silence. Because the apology can be completely insincere and allows you to preserve the integrity and value of your words, and - as the old saying goes, "it's better be silent and have people think you're stupid, than to open your mouth and confirm it."
This image is dedicated to every weak little sissy that I've had to spank for making rude comments about a model. Your parents didn't raise you right; it's not your fault; you're weak and inarticulate. And that's not just my opinion.