I think it’s reasonable to go reluctantly. I think it’s also reasonable to politely decline to attend. Tell your wife and in-laws to enjoy themselves, but politely explain that attending would violate your deeply held beliefs.
We all run into these kinds of situations occasionally. Turning down a meal due to dietary restrictions, refusing to perform an action due to a religious obligation, etc. Ideally, your wife should understand, and your in-laws don’t need to. It really depends what your wife is willing to support.