Bern, it's not that you can't be offended. If you're offended, bully for you.
The problem is that your argument is coming across as somewhat ... tactless at times. You have a claim -- that women shouldn't breast feed in public because it's offensive -- but you haven't given any evidence to support that claim outside of your own ego (that it's offensive to you). And that's making you resort to saying things you don't mean to say (I hope) -- like the zoo comment. You're reaching for a reason to support your opinion, but short of facts, you're left trying to either be funny or to make some sort of emotional appeal.
If you've got real reasons to support your claim that breast feeding in public is socially damaging, by all means, let's talk about it. But if the conversation is going to continue to orbit around "my opinion is my opinion and I don't care if I'm souding a bit sexist" then we're worse than not going anywhere.