Wow.
I would be irritated, but not enough to decline, probably. I generally prefer flats anyway, but none of my flats are formal enough for a wedding. Passive-Aggressive Me would wear flip-flops--after all, they are flats--but Civil Me would suck it up and wear flats, even if they didn't necessarily match.
I think Abby should have touched on the fact that it was a rude request in the first place.