As an outsider looking in and judging based on what they (over)share the whole relationship feels really suffocating. Sure it might not be. But it really feels like it is.
In my opinion, the very nature of his proposal backed her into a corner. It wasn't some discrete, intimate affair where she could say no, not yet, while saving face. I'm sure many people have proposed prematurely, anxious in the throws of an early relationship, drunk on lust and love, with naive intentions bolstered by poor attunement to what a healthy, developed relationship is. However, the partner on the receiving end of those kinds of proposals usually has the privacy to discuss their concerns and say no. The way Joel proposed meant Carrie wasn't in a position to easily correct course if she did feel like it was too soon and say no gently without shattering his ego and the whole world knowing.
There was no privacy to this proposal. There was no space to row back with him with dignity for them both. Everything was incredibly public and he set a runaway train in motion that day which anyone would have difficulty stopping.
Love her or dislike her I think we'll all agree that no one is envious of her position right now.