There is a higher incidence of aggressive behavior between dogs of the same sex than there is with dogs of the opposite sex. Two males or two females can view each other as rivals even if they appear to get along most of the time. This is a fact for every breed btw, not only Pit bulls. However, most other breeds won't resolve conflicts with the determination and intensity that Pit Bulls can. Since we can't let a figthing breed establish pecking order or its own, the hierarchy can remain unclear and cause constant tension between the dogs.
Tension is an important trigger for fights.
We are not saying that you don't know your dog, but we also want to facilitate successful placements. There are other stimulus that could trigger a fight even with dogs of opposite sex. This is why PBRC does not encourage placing Pit Bulls in multiple-dog homes as problems between the dogs may surface eventually. Unfortunately, Pit Bulls are usually the ones people get rid of when these problems occur. PBRC is simply looking to minimize the risks.