Some people say they feel stoned after sex. That is, it's that same feeling they get after smoking pot.
I can't find scientific evidence, but I think this is because marijuana causes an oxytocin release in the brain, just as orgasm does.
Oxytocin is released naturally by the hypothalamus during sex and at orgasm. In the body, it causes a feeling of well-being and relaxation. In the brain, it causes us to feel connected to whomever we're with.
I think this same kind of oxytocin release in the brain is responsible for some of the nicest effects of marijuana, that "mellow" feeling, and also that deep soul connection that sometimes happens, that hippie brotherhood.
The drug ecstasy reportedly produces similar feelings of connection with other. And ecstasy, also known as MDMA, has been shown to increase oxytocin release in rats, while clubsters on ecstasy had higher levels of oxytocin in their blood. I don't think it's a stretch to believe that pot has the same effect -- if your brain happens to work that way. (Some people respond to potential intimacy with fear or avoidance; they may feel edgy or paranoid when they smoke weed.)
So that stony feeling after sex -- again, if your brain works that way, and if the sex felt safe and connected -- would be the same stony feeling you got from marijuana.
See also "The Sex/Food/Love Connection" for insight into how food also triggers these same responses.