A penis is a Fucking weapon. By System:
D&D: The penis does both bludgeoning and fucking damage (possibly piercing, depending). If AD&D first or second, you may save vs. breath weapon (reflex save in 3-4, Dex or Con save in 5). If a monk or enhanced by the 'Natural Weapon' spell, penises are treated as magical (and possibly adamantine for monks) for the purposes of overcoming damage resistance/immunity.
WoD/NWoD: The ordinary human penis does bashing fucking damage, unless the character has certain combat merits, in which case it can do lethal fucking damage. In the case of Gaoru, Kindred/Damned, certain Awakened, and several other supernaturals, it can be used for Aggravated fucking damage with the right special abilities applied.
Shadowrun: The normal penis only does stun fucking damage, but with either magical enhancement, the right cyberware, or being a particularly unusual SURGE, Troll, or non-metahuman sapient, you can do physical fucking damage. Projecting mages could hypothetically do astral fucking damage (dependent on their Astral Combat skill). And let's not get into Virtual Penises...
Ironclaw: Penises operate off the brawling skill (though wrestling may apply in first edition) like any other natural weapon, with fucking damage dice being your body dice. However, a particularly twisted application of a certain Atavist skill can add additional d6s to the damage. Further, there are martial arts in Jadeclaw that may enhance brawling fucking damage. All fucking damage is fatigue unless otherwise noted. On an Overwhelming Success, Penises apply the 'Impale' critical.
HERO System: Depending on the powers of the individual, please consult your scientific calculator for the answer to the fucking damage equation in relation to the defenses against penises. Penetration may apply to fucking damage.
Rolemaster: First, roll melee combat, applying your skill in Penis combat. Then consult the Penis weapon chart to determine whether or not you have achieved a Bludgeoning, Ram/Butt/Bash/Knockdown/Slug, or Acid/Alkali critical or some combination of the above in addition to a Fucking critical. Then roll on each of the relevant tables to determine the outcome of your Penis attack and the amount of fucking damage dealt.