Nobody would describe loose, used socks, drawers, undershirt, and jock strap as a "set of underwear". It would only be a set if it was packaged together in a set, but not even that exists. There are no such sets. You can buy a set of drawers or a set of socks or a set of undershirts, but you can't buy a set that combines them.
Nobody faced with loose, worn undershirt, drawers, socks, and jock strap would type "1 set underwear" to describe them on an invoice.
Only in JFK-land does such a thing happen. Nobody in real life would do it. I said nobody. You hear me, Punk? Nobody.