A couple of excellent blogs such as Rescued Insanity and 24 Paws of Love have recently featured questions on the topic of dog gender.
My own experience these past couple of years has been largely with male dogs, as I've worked with rescue groups and taken in whatever foster dogs most urgently needed taking in, and these have happened to be mostly boys. Whether there's any good reason for that--like whether adolescent boy dogs happen to get into more trouble than their sisters and so are more frequently surrendered to shelters--or whether its just random, I don't know.
My longest-running canine companionship was with a female dog. This was Tashi, my little heeler-chow of 15 years. Tashi was anything but feminine. If there was a cow poop, a dead thing, or anything really abhorrent to roll in, she'd be there. One time when I was camped out with a bunch of activists learning to protest timber sales in Southern Colorado, she rolled in cow poop so vehemently I had to take her to a self-service car wash.
When she would wrestle around in a loving scrap with Lamar, she would be on top as often as on the bottom--maybe out of feminist motives, or maybe just because it was fun that way.
Now Lamar happens to be named after Lamar Latrell from Revenge of the Nerds. I don't attribute this to any knowledge of his sexual orientation, or to anything other than my own warped imagination, notwithstanding all those mornings I've woken to find my sequined jacket rumpled and misplaced and Lamar sleeping like he'd had a rough night.
I've since revised my suspicion that he was headed out to the local gay bar, as he showed what could not have been feigned interest in the little still-unspayed and in-heat beagle mix foster girl I had. All those times with the sequined jacket, he must have had a perfectly respectable gig as a lounge singer.
Aside from Tashi and Lamar's affectionate old-married-couple romps, the most amorous displays of doggie love I've witnessed have been between two boys, my foster lover-men Lars and Fozzie. Those two together were a nonstop humpfest. It was their love for each other that probably saved Lars' life, as I only found him because Fozzie broke out of my yard one night and when I caught up with him, he was panting and smiling with a skinny, filthy stray white shepherd panting and smiling a few steps away. Lars followed us home and the two were inseparable until I found Lars a home with a whole pack of white dogs and a mom who adores him.
I don't really know if dogs have gender identity or if they think of each other, or us, in different terms according to our gender. I suspect that there's not a whole lot of gender sensibility going on in dogs' minds, but I do think that our human perceptions of gender have an impact on our dogs. The Washington Humane Society has a seminar coming up on "The Myth of Gender in Companion Animals and Its Effects on Spaying and Neutering"--what sounds like an interesting exploration of how some humans' quest to define their masculinity through their dogs prevents them from having them snipped.