Congrats on your up coming wedding!
If you get married in Mexico, our suggestion will be to hold a ceremony 1:30 mis. before sunset. At that time, the sun is not strong, the ocean breeze is nice and if your ceremony is facing the ocean it will be not hot.
You have to consider also, if you are having a second ceremony or of your ceremony is at the same resort or perhaps in a Chapel in town, then times will have to change.
Also, if you want to have your photo session prior the ceremony or after ceremony, you always have to talk with your photographer before defining the ceremony time.
Just remember that 2 hours before sunset, sun still very strong and your guests might not want to be on the sun if there is no tent.
Do not forget to consider some bug spray against mosquitos if you are near the mangrooves or during mosquito season (July to September)
On September 2014 sunset is around 7 pm. you can check it out here: