There is a clinical explanation for that and quite true.
When a man orgasms, your body releases a lot of stuff, chemicals and hormones.
Two of them are oxytocin and the other one is prolactin.
It's the way the brain interprts them. Both are related to pleasure and satisfaction.
The thing is that the brain sometimes gets the idea you had enough pleasure so you must shut down... hence you fall asleep.
On the other hand (and pay attention kids) if your body does not release enough oxytocin, it wants more, so you gear up for more action, sex, masturbation... et al.
Same thing with woman.
Some studies say that this might be the key to nymphomania. But it works both ways.
Lack of oxytocin makes their body look for a way to release it, hence a lot of sex.
Excess of oxytocin, makes them feel fully satisfied, but the body again, wants even more.
How's that???
For more information Google: Oxytocin
No, seriously. I bumped into this some years ago and I was intrested a lot with this.
You guys have no idea the ramifications this has in the mongering game and with the girls. Works both ways.
Post below, "You are correct but is just natural after busting a nut and when all the senses come back for both the man and the woman to go "what the hell did I do?"
Oxytocin locks your brain into the "sex mode", blocking practical reason for enough time to do stupid things.
Very intersting subject this... am not kidding.