I could be wrong but, I dont think it is so much if THEY have a male dog. I think it is that most people associate Male dogs as being big, and muscular. Female dogs as being little and petite, cuddly. Which we all know cuddly totally explains being a boxer!! Anyhow... Just my opinion.
P.S. I had a Siberian/Timberwolf mix. Because of her "wolf" markings people assumed she was a he. I gently had to tell them No... He is a she. However when I had my siberian (purebreed) because of the fact she was fluffy, and looked sweet, people got it right, that she was definately a SHE!
Denver