Is it a sin for a man to wear panties and a bra?

Answer 1

No. It's fine, if that's what you want to do.

Nobody has the right to judge you by what you wear - human or religious book.

Whether it is considered a sin depends some on what religion you subscribe to. In general it would depend on the reason for wearing them. If the intent is deceive, then nearly all religions would consider it a sin because it is a form of dishonesty.

Generally it would fall under the DSM-IV-TR definition of a paraphilia - specifically transvestic fetishism. In that it constitutes a deviation from the "normal" means of sexual response, many conservative religions would consider it to be a sin. In Christianity, for example, the Apostle Paul specifically condemned such behavior.