Like I said. I am for having the conversation. There is nothing wrong with it. But it should also be understood that you can't pin everything down, context changes, and the variables are far too great to "frame" as you put it. Whether the person gets offended can change based on the setting, the person saying it, the time of day, the type of day they had coming into the game, etc. Literally, there are countless reasons.
Your phrasing in response to those questions is everything. It is an understanding of how context can change the situation, yet a vague response as to what the DM should do about it. They should try to frame "questions that need to be asked" in regards to each point of contention a player may have. So it is the table or DM's job to sit there and pepper this person with a hundred context questionnaire regarding the thing they are uncomfortable with? That sounds - silly.
But my experience is similar to yours - gaming with friends. Yet, even when I have been the new guy, I've been able to understand the table's boundaries in one session. Most people are probably equal or better at this than me. It's not a super power. And since context changes those boundaries, it is more prudent to know and understand the people you play with than develop some sort of social contract.
I've been gaming with friends for a long time - we don't have the problems expressed in this thread.
But I remember gaming with acquaintances and still occasionally game with strangers at the occasional Gen Con etc. So still see what I'm talking about.