Slingshot: This topic is hilarious...
Both flaccid and erect, a penis is less dense than water. It floats, after all. Hmm, the volume of water displaced by a floating object has equivalent mass to the object. So if you floated your penis on a body of water (say, a completely full bowl of water), and measured the weight of the water that gets slopped over the side, that would be the weight of your penis. Dee should be explaining this all, not me, I'm not being very clear.
The problem is that our penises are all attached to our bodies, which makes getting any kind of accurate measurement impossible. Still a funny thought.