I realize I'm late to the party on this, but I just read through this whole thread and wanted to add my 2 cents on some of the first posts reagrding sexes.
I guess one has to first define what one means by sexes. Yes there are in many cases only two sexes if you define sex by gamete size. Big gametes = girls, small = boys. However, if you look at the body of the organism, then there are many examples of multiple sexes. There are cases in fish (eg. swordtails for all you aquarists) that have males and females, with the males being smaller, but the females can also become males later in life and thus become large males.
There is also fish in the San Francisco bay that has two males, one large one and a small one.