Hear me out:
If you're flaccid, that's your natural weight, as you're just standing there as God intended, no changes to your natural physique whatsoever. BUT, if you're to have an erection, having your dick grow about twice upon it's resting size, and also having it get harder, which means more mass to it, it should implicate, without a shadow of a doubt, that your total body weight would increase, albeit very slightly, no?