Approach someone from behind, and throw a bunch of keys at their feet. As they bend down to pick them up, say "You've dropped the keys to the Gay Club, and now you're picking them up."
This is a better insult than "you've dropped your bender card", because you had proof that they were not only members of the gay club, but also such trusted members that they were keyholders of the Gay Club HQ.
Charlie Webb spots a flaw...
Surely this is ruined by the fact you'd have to - presumably - throw your own keys, thus leaving you to sheepishly ask for them back, after declaring them gay?
You : No, seriously. Can I have them back? I need them.
Them: You need the keys to the gay club?
You : No, they're my house keys.
Them: YOU LIVE IN THE GAY CLUB HQ?
Potential nightmare.