This is a HUGE pet peeve of mine. It is a joke, and my wife knows not to ask me either. Jack & Jill showers???... Jack & Jill bacehlor/ette parites??? No freakin' way!
The charm of fishing is that it is the pursuit of what is elusive but attainable, a perpetual series of occasions for hope. ~John Buchan
so just say "no" and listen to the yelling as you pack you fishing gear and be done with it.
And you can let her know that to avoid this type of disagreement in the future that she shouldn't even consider asking you to another jack and jill event and that because she asked you this time you can politely inform her that this argument is all her fault.