You feel more exhausted when cum leaves your penis.. so does a girl feel more energetic when you cum enters her cunt? Seems logical considering the "conservation of energy".
As much as this is a joke, there is actually some truth to this.
There has been a recent study on sexual satisfaction amongst couples and that women generally perceive a higher level of satisfaction from men who do not use a condom and ejaculate inside the vagina.
The reason for this is due to the residual testosterone contained within semen entering the woman's bloodstream.
I tried Googling for the news story but as you can imagine, entering the necessary keywords into the search engine results in a tirade of unrelated shite.