Even in the midst of a drought you can still be rained out. However unlikely you think it, you must have a wet weather alternative. Putting up a marquee at the last moment is just not an option. Talk to the venue for your reception. Some venues will be happy for the ceremony to be there if it is wet.
Delegate the decision to relocate to someone apart from the bride and groom. Parents often are very helpful here. The bride and groom have enough to think about and should leave this to someone whose judgement they trust. Whilst I very rarely have been rained out, some of my most joyful, happy and memorable weddings have been quickly relocated because of awful weather. It is great to have the ceremony in a place you have especially chosen, but it is the couple, the guests and the way the celebrant manages the ceremony that creates the joyful atmosphere, not the scenery.
And did you know that there is a superstition that it brings a couple incredible good luck if it pours with rain? Who cares if it’s true? If it rains for you, just believe it.