I would recommend where I got married!
The Gran Bahia Principe was a perfect fit for our guests and it is usually very reasonably priced.
all inclusive - Indeed! I LOVED the food! Yummy
all ages - We had children with us, actually two babies who were 1 1/2 years old and they LOVED it. The pools actually have a shallow area that they could sit and play in. It was perfect! In addition, they have a kids area and lots of animation staff that love children! It really was prefect for the kids.
beach wedding (actually on the sand) - My feet were IN the sand during my wedding! It was amazing! And it wasn't a swimmable portion of the beach, so there were NO bikini clad people staring at me and in all of my photos. Instead it was on the beach with the beautiful blue water crashing on the rocks in front of us. Perfect, if you ask me
around $1500-$1600 for couple 4 days - The GBP is typically around this price including airfare. There are three resorts, (Tulum, Akumal, Coba), and Coba tends to be the cheapest because it's NOT on the beach, however you can use the facilities at all three, so it's not a big deal.