Protest what?
I'm sure the players are capable of having beers without needing to wear a costume. It's just not for me. If other people enjoy it, cool, I just would feel uncomfortable.
I only relate from the experience of going to a party with a fancy dress theme one year and being one of the lame ones who stood out at being in the handful of people of a crowd of 100 who didn't make an effort. Killer party!
Really doesn't hurt to spend $20 on a superman/wonderwoman costume at a local hire shop just to get into the spirit of things - if you're lucky enough to be invited to one. Oh and I was 10-15 years older than many of the fellow guests, so you're only as young as you feel.
Try it someday, you might enjoy yourself!