A previous thread on symmetry: Why is there symmetry in life?
Evolution doesn’t plan for things. The simple answer is that two-balled creatures survived until procreation more often than single-balled creatures. This tells as that there probably is an advantage.
The reason why we don’t have more than two is because redundancy has to be balanced by efficiency. If we had ten balls, that’s a lot of extra enegery that needs to be spent by your body to grow them, keep them supplied with nutrients and have them all producing sperm.
So, what you’re saying is, there’s hardly enough blood and nutrients in my body to support one penis, let alone two. 
Well, some reptiles (snakes, as I recall) DO have two hemipenes. And male sharks have two claspers. We mammals just got shortchanged in the male equipment department.
I believe that the plural of penis is penii, Bearflag :). Or something like that. Anyone else in favor of abolishing the alumnus/alumna/alumni/alumae for alumn/alumns?
Two balls? I dunno. Redundancy in the case of cancer is not a good argument, because of the rareness of cancer before breeding age. Actually, now that I think about it, the more equiptment you have, the more chances of something getting mucked up (what about the mouth-nose-throat-esophagus-lungs situation? Choking is too easy).
I believe that we have two balls just becuase that’s the way it’s worked out for a couple million generations - no strong reason to change it away from the bisected ball-sack between the legs.
I mean, it would seem easier to reqork the way sperm are made so’s to put the male organs inside the abdomen.
Bilateral symmetry, yes – but there’s more to it than that. Most glands provide something essential to the ongoing health of the body, and therefore they’re either (a) fairly large (thyroid, pancreas), (b) well protected (pituitary, pineal) or © reduplicated (adrenals, parathroids, ovaries, testes. I think the assumption is that there’s a distinct evolutionary advantage to having two in case one becomes defective through injury or disease, but that there’s insufficient advantage to having three or four over two to have caused a third, medial gland or a duplication of the two glands to occur – and note that this does have an exception – we have four parathyroid glands, their hormonal product being essential to survival.
Penes I think.
The OED lists the plurals as being:
penes, peni (erron.), and penises
**Why two testicles? **
Because saying “You have the BALL to say that??!” just doesnt sound right… 
but for those foisting bi-lateral symmetry, how come we have only one liver, one spleen, 2 kidneys but one bladder nad have only one heart and brain?
…just pulling your bi-lateral chain… 
Species with one teste got wiped out somewhere along the line by a vicious kicking match.
[Trekker]Yeah, they have all sort of failsafes and stuff. And if I rememeber right, they might even have a third testicle. Some of the books I read are rather… er, uh… detailed.[/Trekker]
There actually is a reason for that. It’s been too long since my “Vertebrate/Invertebrate Biology” class, but IIRC, it has something to do with which cell layer they develop from.
Yeah, but on the other hand, Sharks don’t actually get to make physical contact with their mate, so they don’t get to have any fun at all.
Not sure about reptiles, but I wouldn’t want to be one of them either. I have enough trouble getting out of bed on a cold day without being exothermic.
I actually covered this in the thread I linked to earlier. Those items which we have one of tend to develop from endoderm, while the bilaterally-symmetric organs develop from mesoderm and ectoderm.
Correct. As do lizards and crocodilians. And, despite the “hemi” prefix, each is fully functional, independently of the other. Often, both are used during one copulatory session, one at a time, of course. Some male snakes exhibit sexual stamina that human male porn stars can only dream about.
guys, you add in undescended testicles, cancer, blocked ducts and plain old torsions, and i reckon two is a good safety factor.
(actually you only need about 1/4 of one testicle to still be functionally fertile, 2 is actually overkill)