Ceallach, I read the OP's first, and especially second post differently.
She didn't say that her SO could make the decision, she said that he should make it. I think she made it clear that she wasn't ok with him breaking their plans. He seemed to be dithering, beating around the bush, and trying to get her to "give him permission"
So while the OP may not have been communicating 100% effectively, I don't think it's fair to say that her SO was "blind-sided" as he seems to have some trouble with clear communication himself, and it seems that he knew she was unhappy with the situation.