why do you think men and women don't go to prison together?

So prison is supposed about repentance and rehabilitation. If men and women were placed together I couldn't imagine there would be much need to focus on those things. I mean especially in the instance of boredom and nothing but time and a preference for the opposite gender.
If you're suggesting a male side and female side. I still think they would find a way to communicate, fornicate and god knows what else.

I think I read something recently about two inmates that managed to conceive a child through an air vent despite never meeting.

I think it's hard working in a prison, but this would make it 1000x harder.

Prisons tend to have a “Lord of the Flies” aspect as they currently exist. I can only imagine how much that would be amplified with coed prisons. You probably couldn’t pay the guards enough to deal with that environment.

I’ve been sentenced to do time, that’s my time and I don’t want a woman annoying me

They can very well be incarcerated at the same time, but prisons are segregated!!

Too much hanky panky

Because there is a law against cruel and unusual punishment.