You will need to go to the mainland. I can speak for Placencia as I have recently returned from Maya Beach. Marti & Eldon held a wedding at their Singing Sands Inn, and it was fabulous, although smaller than 50. 50 gueast could be accomodated in walking distance of the Singing Sands, although that many would be a huge undertaking for any resort on Placencia[i don't know them all--maybe Luba Hati is big enough] Maya Beach would give guests a range of lodging choices, and is very romantic, yet ruins, snorkling, diving and the rest are all available.
If Maya setting is a must, then maybe north to Altun Ha/Maruba lodge or inland to San Ignacio.
I know Maya Beach would be lovely for a wedding and is diverse enough to please all but the most picky traveler.