Well Clark's skin is strong as steel, but he's not "STEEL SKINNED" hence why he can kiss someone and his lips aren't like two pieces of stone or how he can lick an ice cream cone and not send the scoop flying with the first lick.
Or when he does partially lose some of his invulnurability his blood doesn't shoot out of him like a shaken soda can. I mean how fast DOES human semen go anyway? Its a squirt, sure some guys might really shoot it but c'mon... even taking into account his super fast system, every breath he takes doesn't suck all the air in the planet in and out with each respiration. So you're telling me that even the fuckin' act of BREATHING is something he has to control? Super-breath is just that, its super its something he CONCIOUSLY has to control. Sure from time to time a sneeze will create a sort of quick burst of air, but its hardly enough to literally blow out the side of a building.
And the whole "machine gun penis" oh please guys and gals... if he had that kind of offensive weapon on his jock... that's just fuckin' scary. Seriously. So what? When he pees does he break the toilet too? Or has too much mexican food does his crap fly out at such a rate that he has a six pack of toilets from Home Depot in the closet, just in case?