We had the same problem when our boxer reached his first birthday; the good news is that by the time he was 2.5 all signs of this behavior were gone.
To share a story, I took 18-mo. Jack to PetsRUs for some "controlled socialization" when a man in his 60s approached Jack with his two grandchildren. One a boy and the other a girl, the two appeared to be roughly the same age and both acted in the same calm, friendly, curious manner to the dog. Jack immediately steered clear of the two males and approached the little girl in a very open, friendly, and sweet manner then turned over on the floor to let her scratch his belly. As both the older and younger males tried to show affection, Jack bared his teeth and acted very aggressive and skittish at the same time. All of this occurred in the matter of two minutes. It was VERY apparent that he wanted no part of anything male.
What we suspected: 1) he had a couple of mildly bad experiences with two different men at two different kennels (one of the two was bad enough that I refused to consider taking him there again) just prior to the onset of the behavior. I do not think that these were the provocations of the behavior, but rather helped to intensify it.
2) I am Jack's primary care giver and my husband is unabashedly afraid of dogs (a fact I couldn't truly believe until AFTER Jack had been with us for 8 months.) So most of Jack's early dealings with men were awkward at best. (My husband's entirely family has an irrational fear of dogs and each of the 5 men and one woman in the family displayed fear and difficulty with the dog.) All of this solidified his distrust for men. I think that boxers are profoundly intuitive and open dogs and it doesn't take much when they are puppies to affect their behavior going forward. Yet, as they are easily affected, they can also be trained and steered out of bad habits.
If I had it to do over again, I might not introduce a dog to my husband (very sad) especially not a big dog like Jack, who frequently showed my better half who was really the boss. Despite the fact that Jack, the dog, is afraid of his own shadow. We did wait it out and everything turned out just fine but it took a year of my running interference for Jack when men were around. This of course took patience and a combination of kindness and tough love directed at the dog. He is an amazing comfort to me now! ... View related article.
