First, you're wrong....the worst words a photographer could hear are...."I just accidentally dropped your camera down the outhouse seat and it sank out of sight....sorry!"
Second, like any good wedding photographer....
--set expectations upfront
--get examples of what they want (both in terms of specific shots and also style or look/feel). Are they looking for a story (start to finish), a traditional, an artistic, or a romantic set of results?
--put together a shot list.
--backup your gear. Have extra SD cards, extra battery, extra speed light, extra body.
--visit the site ahead of time. Chex out the lighting. Figure out where you need to stand and when.
Third, if you really, truly, absolutely want to get out of this assignment, then you tell them the truth. You say "well, I'm willing to do this but I was so looking forward to attending the wedding and now I can't." They'll look puzzled and then you explain (honestly): "if you ask me to shoot the wedding, you're telling me I can't participate. I can't give toasts, I can't join the drinking, I can't throw the rice, I can't listen to the ceremony, I can't listen to the musicians, I can't hug family members, I can't enjoy the moment. I'll be working. I'll be focused totally on details like ISO and exposure compensation and depth-of-field, and keeping my lens clean, and getting all of the wedding party lined up and in sequence and yelling at people who wander off to hug the groom or grab a drink. I'll need to be keeping track of everyone I've taken photos of (and people I haven't). If I'm photographing the wedding, then I'm not participating in the wedding, I can't be a family member. For that time, I need to be a professional willing and ignore all of the sentiment and joy and special moments, be willing to order my elders around, walk in front of people viewing the ceremony. I'd really like to be there for the wedding but if you insist I won't--I'll just photograph it."