Theyre just theories regarding reproduction not the actual division of the sexes.
If evolution was to occur it would require a mass 'evolving' of one species into another, otherwise they would not be able to reproduce because of the differing amount of chromosomes.
How did asexual single cell creatures evolve into other creatures and how did they become male and female?
Evolution occurs, at a basic level, like this. An organism (let's say a human) mutates slightly. Let's say that mutation is a genetic mutation in the immune system that provides immunity to a certain viral infection but also causes blood cells to take up less oxygen. That mutation means that the person with it survives the virus and lives to have sex and pass on their genes. By passing on their genes, their children also have that mutation.
Their children survive the virus we spoke of and also pass on their genes. The depleted oxygen resourcing means that the child has less oxygen in the bloodstream and the body responds by expanding the lungs farther. Eventually, this mutation is passed on to a point where the organism in question has bigger lungs than other humans that don't have this particular mutation.
The human with the genetic disposition for bigger lungs becomes athletically superior to those humans with smaller lungs, and mates with humans that don't have that disposition and fathers or mothers children with larger lungs. Now these organisms have two differences from the other humans: immunity to the aforementioned virus, and superior oxygentation via larger lungs thus superior athletic ability.
They are, technically, a very slightly different organism from the ''normal'' humans, who can still mate with the ''normal humans''. But a problem arises, the increased oxygenation leads to hyperoxygenated cells,thus the body has to adapt for the oxygen increase by growing bigger muscles to use up all that oxygen, thus these humans now have superior muscle mass, superior lungs, and immunity to a virus compared to the ''normal humans''. As you can see, the organisms are getting increasingly different, step by step, generation through generation, to the ''normal humans'', but their reproductive process is no different.
Eventually, their reproductive process may begin to change, small steps at a time. By the time that change comes around(if it ever does) there will be generations upon generations of these ''superior humans'' around to be able to mate together.
Apply this kind of slow-progressive logic to every evolutionary process you can think of. It doesn't happen overnight, and if an organism mutated to a point where it COULDN'T reproduce with the other humans, then it would simply die off and would not have had the chance to further that mutation by reproduction, since it can't reproduce. So, in summation, the mutation only gets passed to the next generation if procreation happens.
As for how creatures first evolved the ability for procreation, it evolved as a means for simple eukaryotes to avoid deleterious mutations, pathogens, and to increase the rate of adaptation. So, in otherwords, simple celled asexually reproductive organisms would be threatened by viruses, for instance. The rorganisms would have responded mutationally by the development of sexual characteristics. Think of a virus. It attacks the host, and the host builds immune defences, but because the host reproduces asexually its ''offspring'' have gained no NEW defenses against the virus, so eventually the virus wins out over the host. By splitting into separate ''sexes'', the organism can increase its chances of mutating new defences against the virus, because when the two sexes procreate together they mix their varying genetic material in ways that asexually reproductive organisms can't. They increase their gene pool, taking on mutations from each other rather than relying on an asexual lineage with limited mutational capability.
In fact, the best defence in response to viruses would be polysexual reproduction, or reproduction from three, or four, or five or six parents -- the more parents the better to fight the viruses and pathogens.