I'm a day late and a dollar short on this, but I thought I would chime in and mention that our rescue also has the policy of not placing two female Boxers together in the same home. I get calls all the time from people who purchased two female littermates who then hit sexual maturity and try to kill each other. The tough thing is that females will sometimes get along for weeks, months, or even years, but when they start to fight (if they do start to fight) they will not stop. Males can often have a tussle and then get over it, which is why we do sometimes place two males together in the same home *IF* the circumstances and environment are just right.
I have a female Boxer myself and it's not a myth that females are stronger and more driven in a lot of ways. I fostered one female that challenged Lucy and oh my! The two girls got into such a fight. One bit the other in the ear and blood flew everywhere. I thought I was living in the Amityville Horror (remember that movie?).
But the main point, and others have made it very eloquently, is that as a rescue we have to do everything we can to make sure that an adoption is successful. Since the Boxer breed is no stranger to inter-bitch aggression, our best bet is to place a female in a home with either a male, or no other dogs. That is not to say that some people can't have females living in harmony. I know people who've managed it just fine.