LOL "I'm down [for it]" is an expression of desire to participate, i.e. 'Put my name down on the list', 'Sign me up!'. The opposite would be, "I'm bummed." "bummed out", "it's a drag", "I'm out".
Dude, you probably don't give this much thought but I spotted your hot little uncut nub in the thumbnail and clicked on it. Sure enough, you completely ignore your dick you're so enamored dousing yourself with your yellow -- it is *yours? or is it a friend's piss? It would be hot to watch a closeup video of it. And would also love to see you give your pisshole and bladder a piss enema!!!