All public marriage proposals ought to be accepted, just out of politeness, even if the askee doesn't want to marry the asker.
You can imagine how devestating it would feel to go through all the trouble of arranging a public proposal, a magic moment, only to be rejected. In front of witnesses. It is something that would haunt you for the rest of your life. You certainly couldn't go back to the returant again.
It is far more compassionate to say 'Yes!', smile, and let the asker have their moment. Then later, even the next day, you can express rervations. 'Cold feet' and 'going so fast' are good phrases to remember.
If you still want to consider a future together, find a reason to set a late date. If you really think you want a future together, be honest.
If you were having fun, but were put off by the unexpected seriousness, then ask for a lot of space to gather your thoughts. Just don't take too long in breaking things off entirely.
What you should not do is be honest at the moment you are entrusted with someone's potential public shame and humiliation. That brutality should not be visited upon those whose crime is adoring you.