Sex? How many is best (I'm talking about genetics you pervs)

Music: Os Mutantes (heh, apropriate)

So, I was just over at the parent's house having dinner and watching firefly as we do every monday (and by every I mean two of the last three mondays and one sunday), and a large part of our dinner time conversation was about getic evolution and specifically why it is advantageous to have two sexes instead of just one or three or more.
No shit.
The conversation just naturally turned to that.
I think it start talking about internet memes, then that went to how Darwin came up with the term meme, and that led to why certain traits get involved. Then we talked about computer simulations to demonstrate evolution and how if they are just a couple simple rules, they then follow the same basic paths of cells and develope new traits and even organisms. And they will even change from asexual creatures to ones with two distinct sexes. But we couldn't think of why.
At first.
We could understand that more than one sex is better than just asexual reproduction. But we couldn't think of why two sexes is better than three (and it would seem that way cause there are no trisexual organisms), or even for that matter why there are both male and female sexes in many species. Why was this better than having two equal creatures that still shared they DNA without having one male and one female organism. But then I think we managed to work it out.
If there are two creatures involved in mating, one of them must be the one to give birth - but which one? The one that gives birth has a higher mortality rate, but it would stay evenly split until some started to mutate a trait that cause them to be less likely to be the childbearer. We'll call this the male (although at this point is is still childbearing, just less likely to do so). Since the male is less likely to bear the child, it has a higher survival rate and will over time become more and more likely to be the non-childbearing parent until eventually it will just is no longer able to bear children. This mutation would be so succesful compared to the child bearing parent that the concentration of non-childbearers would grow higher and higher compared to the childbearers. To a point.
Eventually there would be so many "males" compared to "females" that the males would start to decrease in population as there are too many compared to childbearing females to pass on the male's genes. The population would waver back and forth until it eventually reached an optimum equilibrium point and they balanced out. So, a male by itself would thrive as a sex, only so long as there are still enough females there to give birth to new males. It eventually forms a symbiotic/parasitic relationship between the two sexes.
And so now I am done with my crazy evolution blathering. If you enjoyed it, good for you. If you didn't well, you coulda left, so whatevs.

... man, krista is totally not gonna let the fact that I just refered to males as parasitic go.

Assuming she read all the way to the end. We shall see...