Well, there's still exceptions to the rule; namely, if terms aren't commonly understood. For example if the person asked thinks that, "No I'm not okay with that," means that they withdrew permission and the other person will have to cancel. "Are you okay with that" sounds more like a negotiation than "May I," however misunderstandings may still sometimes take place.
Does that make sense?
I don´t think the other person HAS to cancel. It´s still their decision - a permission was not asked, only "are you okay with that".
As KC43 stated, and I agree, eventhough I hope we can work on someway of helping the person having issues (in my relationship, that´s me) feel okay with that (and, honestly, I think even only asking already helps):
Naturally there might be times when there is no compromise, or no way to make the "not okay" person feel more okay, but in my experience at least, the discussions almost always go the way I've written. (And there have been a couple of times when in response to my "does this help" Hubby has said, "No, not really, but that's my problem and I don't tell you what to do, so go have fun and don't worry about me.")