Would this be rude or warranted?

My husband and I agreed to pay for 1/2 of my step son's rehearsal dinner (his mother responsible for the other 1/2). We are vegetarian and have made our feelings known that we are not happy paying for meat. His son and future wife have not provided any information or allowed us to be involved in any of the planning process - just being blown off. If he hosts a dinner where we are unable to get a decent meal out of, would it be appropriate for us to just leave and go out to dinner ourselves? (my husband is borderline underweight and I am hypoglycemic so a late meal or missing a meal is not good for us). Note: they KNOW our status, so it wouldn't be an accident.