One guy posted on the board I got the link from claimed it was because we were exhausted after all the crap we had to go through to get them in bed.
He does if I've done my job right. If he isn't sleepy, walking with a limp and mentally muddled, I feel I've failed.
I'm sure I heard once that it was a pre-society thing, designed to give the female a chance to get away... No idea if that's true or not, and I have to say I can see problems with it - make the male more vulnerable to predators, for a start.
depends on the [-]hand i'm using[/-]person i'm with, i mean with the right girl i can quite literally go all day and all night, whilst at other time its been a 5 minute hump, spurt, sleep. and there was a great summers day that consisted of shagging and catnapping in an hotel room. that was exhausting, and bit on the magical side.