"Why is it after having sex, most men want to either leave or have their partner go away?"

Its because you've just given up all your bodies energies by ejaculating. If you go full blown orgasm and ejaculate you drain your energy, your chi, and all you do is want to shut off and sleep. Your body gives so much of its energy when you ejaculate, which makes sense as your creating a new life. The trick is to learn to become multi orgasmic (yes its possible for men too) and separate orgasm from ejaculation thus not wasting your chi. When you learn this you can have sex for hours, and orgasm many times, and still feel 100% energised afterwards. Its explained fully in the book "The Multi-Orgasmic Man", im about half way through it at the minute ..

http://www.amazon.com/Multi-Orgasmic-Man-Sexual-Secrets-Should/dp/0062513362

I don't think this is a rule.  It probably depends of the individual.  I feel pretty refreshed right after.

Precisely needs_and_wants!  Hence the Taoist practices I mentioned earlier. 

Personally, I limit myself to very few ejaculatory orgasms per year now.  There are a whole host of books on the subject but I prefer the more traditional taoist reflections and advice rather than some of the modern sensationalist, almost consumerist books on the subject.  Done correctly, chi can not only be preserved by certain practices but actually 'gained' during sex with the right partner from the females multiple orgasms! 

I dont know what you are talking about mate. After those backbreaking 2-3 minutes of labour, my woman falls asleep and I start up Cubase. Seriously though. The chemical balance in your brain changes after sex and it does stuff to you. Some want to sleep, my creativity explodes so I want to produce, etc. But I think that's all there is to it.