I fish a small river and I have noticed that if I catch two fish out of a hole it is almost always male and female , never two hens, some times two males .Is that just by chance?

I've found much the same thing, generally what is happening is the bucks are looking for the hens and when they find one they stay on her tail. Usually there are more males than females and several bucks will be attracted to the same hen - the big one will hang around closer and keep the little sattelite males away. Bucks are more aggressive and usually bite first if you are using an attractor type of lure, and I find that if I use eggs or something similar I will get a hen to bite first.