Ok. My opinion, for what its worth.
I've got a 7 year old male Newf X.(Cabeza) He gets along with EVERYONE. When we introduced an 8 month old American Bully/Pitbull into the house, also a male, there were no problems at all. And the Bully was not neutered at the time. They lived perfectly harmoniously until Bo went to heaven at age...um...1 and a half. I know, not very long, but the point is they did well. Not even any introductory B.S.
That said, we used to have a male Husky. When we introduced Cabeza into the house, the Husky could NOT deal with it. He became INSANELY aggressive. Cabeza had no issues, but the Husky did.
There is something super cool about the mixed gender relations, though. Jenn's right. My guy really loved dober_gurl's Dobie, Maggie. He totally acted different around her. Usually he's all aloof and doesn't even pay attention to any other dogs, but he really loved her!
I think that it really does have alot to do with the personality of the dog. But, I personally really like the boys. Maybe I'm too much of a B*T** to share the home with a girl. I've never tried, though. And, I've always said that when I finally get that Newf pup, its going to be a girl..so, we'll see.