Bertie - I get what you're saying, but I am a people-pleasing conflict avoider, and I still have rock solid boundaries. At least when it comes to relationships and friendships (and I'm assuming that's what this discussion is actually about).
To me, it's not about being OK with the concept of upsetting people.
I've never needed it to get anywhere near that far.
I guess my dodgy man radar is so well-tuned, that I'd never even get beyond small talk stage with such a person.
I don't have to be rude to anyone, because I zone these sorts of people out automatically - without even really being aware of it.
Same with friendships. I've never had friends take advantage of me, treat me like a mug, or take, take, take - the way I often see written about on here. I don't need to put my foot down, stand up for myself, confront anyone, etc. It never gets that far. This is what I mean by never having really even thought about my boundaries. They're just there.
I feel really wary about saying these sorts of things, as I don't want anyone to think I've got it all sorted out, or worse, that I'm victim-blaming, or insinuating that people who can't do this, are somehow responsible for bad relationships, etc. They're absolutely not.
I'm just lucky because of my background, upbringing, parents, etc.