Wow! I would definitely get a female...actually I would NOT get another dog involved, but if you have to get another dog, definitely go with the opposite sex. I've seen too many same sex dogs (especially two females but also males) become problematic as they reach adolescence (2-3 years old) and it can get ugly. But I suppose genetics also have something to do with temperment...that's a toss-up. Knowing what I know now, I would keep my boxer as my ONLY dog.