This is not a problem. May I suggest the following:
The "I think I'm going to have diarrhea" excuse.
The "I think I'm going to have diarrhea" excuse is well known for it's fantastic results upon being used when pulled over by a police officer, getting out of weddings, plays, reunions, etc.
Let's face it, the Misses would rather you stay home and not embarass her in front of her family and friends if you actually did get diarrhea. It's not like say, a headache where you can just pop some Advil and pretend everything is okay. Trust me, she'll want you to stay home.