A: It's up to you to be the mature one in this testy triangle. Did Mom leave Dad for the new guy? If so, his presence may embarrass your father, so ask Mom to leave her boyfriend at home. Did your parents split a decade ago? Talk to your father about his ultimatum, which comes across as unreasonable. If he holds his ground, consider alternatives. Maybe Mom can help financially, or you can downscale to a wedding you can afford on your own. Dad can always throw you a smaller party after the honeymoon.