Please embrace it. Some of my favorite shots I've ever taken were at rainy weddings. Hopefully, if you're expecting rain, your venue has a covered/indoor option. But if it doesn't, be sure to remind guests to bring an umbrella, just in case! I've shot ceremonies that took place in the direct rain, and it may not be what you had hoped for, but it's not the end of the world. And it certainly was beautiful. You can't control the weather, but you can control your reaction to it. So don't let bad weather ruin your wedding mood. Embrace it, have fun, and spend a little time getting creative rain photos. You won't regret it.
The weekend that Columbia flooded in the Fall of 2015, my company covered 4 weddings. We got some absolutely amazing photos and video, and luckily all of the brides were very happy! At one wedding I photographed, the couple expected rain and had no indoor option. So what did they do? Brought a slip-n-slide! While I don't expect most brides to be that flexible, just remember to embrace whatever comes your way!