Sexual reproduction evolved a long time before mammals came on the scene.
I would think that the primary advantage is increased genetic diversity in a species.
I am talking about way before mamals existed. I am talking about when the first "living" organism decided to move away from being unisex. Organisms such as bacteria.
The question is, which came first. I ask this question from a scientific point of view. Has it been determined through fossils, if the first organisms were unisex, male or female. (as we define them today). From my research, they were unisex or asexual. Even though these single celled organisms (such as bacteria) reproduced asexually, they developed more complex reproductive structures to help with the dispersal of the newly formed organism (cell splitting).
To me this indicates, that there were first unisex organisms, then female organisms who developed mainly becuase it was easier for them to disperse their offspring, and in all likelihood, the unisex organism, then developed into the male organism.
So my conclusion is:
The reason I am even contemplating this question, is that it makes me wonder if men and woman are not their closest relatives. I know men and woman are classified as the same species, but woman seem to be an evolution from the unisex organism, and therefore technically , woman are mens closest relatives and vica versa.
This is why men and woman compete with each other so much. It is our instinct, it is part of a continuous evolutionary struggle.
I think this is what Rob meant when he said
Dual sexes has to do with genetic variability and competition.
The struggle between the sexes, seems to have had a major impact on the evolution of humans.