jdman,
I'm not sure why some of the previous responders felt the need to be assholes because this is a legitimate question. Where else can you go to ask?
Cumunion is not a holy place. It's where guys go to fuck. They do provide rubbers, so asking what the odds are of guys fucking you with rubbers is appropriate. (It's always low odds and very rare at a fuck party.) Check out PrEP. Also clarify with your gf the level of risk she's willing to accept, including HIV and all other STIs, and the level of risk you're willing to accept for her extracurriculars, including disease and pregnancy from a random guy. Just because you bb with one guy doesn't mean you're required by the Laws of Gay Sex to bb with every guy.
Dan Savage has covered open relationships and his site may be helpful.
Good luck and happy fucking!