^^ what he said. Its their day not yours. It might be surprising because the general consensus on this forum is that the majority of the public does not know anything about clothing, but i went to a friends wedding recently where everyone was in suits. I wore a solid navy blue suit from Steed with a white shirt and grey tie, pretty much what everyone else was wearing but swap navy blue with grey, and the grey tie with blue and vice versa. The bride actually approached me that night and asked me why "i out dressed the groom" I felt like a jackass the rest of the night. Now would i go and buy a cheap suit for these things? probably not but in the case of the OP he is actually being provided with what to wear,
that would make me feel like crap as well.....although at the end of the day is not your fault, specially if you are a guest, but to be in the wedding party, you must obey by the grooms and brides rules, funny thing is im sure women who have been bridesmaids have dealt with worst than this.