That got me thinking. (And, I suppose that thinking can be sexy… meow…) Is it the biggest?
First, let’s take a step back and consider what it means to be the biggest. You can argue that this can be measured by either weight or volume, but luckily the two will wind up being about the same when we’re talking about human soft tissue. Human density ranges from around 0.9 g/cm^3 for fat to around 1.1 g/cm^3 for brain tissue. Muscle weighs in, literally, at 1.05 g/cm^3. These are all very similar to the density of water, which is convenient for this thought experiment. Bone, on the other hand, tips the scales at 1.5 g/cm^2 — so unless you consider bones sexy, I’m going to discount them for now.
Looking at Wikipedia for the average size of brains it comes to around 1.3kg. Adult males have slightly larger brains, females slightly smaller. I do want to stress this is based on the differences in head volume and not intelligence; we’re comparing size here.
Is the brain the biggest sexual organ?
Well, depending on your cup size, no, it’s not.
If you’re counting “boobs” as a single organ (debatable of course), if you’re cup size is at least 44A, 42B, 40C, 38D, 36E, 34F, 32G, 30H, or 28I, then, in fact, you have more breast tissue than you have brain tissue.
If you insist on counting each breast on its own, then the cup sizes you’re interested in are 44E, 42F, 40G, 38H, 36I, 34J, 32K, 30L, or 28M. These are certainly some big knockers, but they exist out in the wild, and more often a bit surgery may have been involved in their creation.
You can argue that “boobs aren’t a sexual organ, they’re for feeding babies!” Well, that’s true on the face of it, but you must admit to the fact that there is a large population that finds them quite a draw. The fact that human females have engorged breasts throughout their adult lives is very much an outlier among the primates. They are one of the defining secondary sexual characteristics of being a human female. Brains, on the other hand, are possessed by everyone.
Are brains the biggest sexual organ? Well, it depends on your cup size if you’re a woman.
For men? I would argue that one of the principal secondary sexual characteristics of human males is their musculature caused by the added testosterone. This also weighs more than the brain by far. But it’s not nearly as funny to talk about in my opinion. :-P
Then there’s also butts.
It’s harder to get a volumetric assessment of butts though.