Getting proposed in front of a room of people can be deeply romantic, if it's something your partner knows you're on board for. But sharing that very intense moment in front of an audience isn't everyone's cup of tea, particularly at another family's wedding reception.
While there's no hard and fast rule about what constitutes an appropriate proposal setting, it's fairly common sense that you don't propose at someone else's wedding because it's a major spotlight steal.
"AITA for intentionally ruining my BF's proposal?"
"BF and I were at my father's wedding reception, and my entire family and dad's friends were there. BF been dropping hints all day about a surprise he had planned and he kept putting his hand in his trouser pocket, but when his hand was out of his pocket I saw something that could have very easily been a ring box."