Everybody needs a hobby I suppose.
With the hundreds of thousands of autopsies and pornos, I think a better question is: Why hasn’t the fluid ever been lab tested? It’d settle the urine question pretty conclusively. And why hasn’t the fluid sac been found in any bodies either? It’s got to come from somewhere.
Oh, it’s been studied extensively. The studies just show up in the medical journals. A very nice recent paper did a systematic review of the literature.
In that paper, there’s a table that breaks down the different fluids that we’ve been discussing: voided urine, squirting, and female ejaculate. There is usually a large quantity of fluid during squirting; one paper reported 50–900 mL of fluid, which is perhaps comparable to an amount of voided urine. However, there is generally a very small quantity of female ejaculate. The physiological components were studied with ultrasonography and urethroscopy.
The paper categorizes squirting as clear fluid, and ejaculate as white. Among the most commonly seen components of these two fluids are: PSA, PSAP, glucose, fructose, sodium, potassium, chlorine. Squirting has uric acid, creatine, and urea whereas ejaculate doesn’t.
Anyway, it may be worth a trip to the library (Z. Pastor. Female Ejaculation Orgasm vs. Coital Incontinence: A Systematic Review. J of Sexual Medicine. 2013).
Didn’t know that, thanks for the cites!
Sounds like a novel way to encourage more plasma donations! 
(Note: Quoted post is from that other, previously linked thread. Just figured that I’d reply here since this thread is already bumped.)