[CrochityOldMan]

I have wondered about how sex evolved for quite some time, if anyone could shed some light on this issue it would be appreciated, because it seems to be a legitament problem.
According to the current theory of evolution the first organisms reproduced asexually, so at some point organisms must have developed the ability to reproduce sexually. I know there are some explanations at a unicellular level of cells connecting and exchanging DNA, but on a multicellular scale there seems to be a large problem. In order for sexual reproducation to be possible it takes a male and female, so a male and female would have to evolve seperately(while reproducing asexually) to the point where two of these different organisms get together and decide to reproduce. It seems very improbable that two such organism would evolve seperately and in such a compatible way. A completely worthless penis would have too be grown and mainted for generations without haveing a purpose, and then be lucky enough to run into a female that had developed an equally useless organ set over an extended period of time. How is this possible?
I know that many plants are both male and females and this seems to get around the problem, im not sure if there are any such examples in the animal world that have the ability to reproduce with themselves and with others, but even if there are, why would you ever lose the ability to reproduce with yourself? as it is obviously an advatage to have both options avalilable.