Theories for the origin of sexual reproduction are many and rooted in bacterial reproduction. It would take a full semester to list them all. Most point to selection for diploidy and recombination as as means to correct DNA duplication mistake. Long term diploidy and sexual reproduction (as defined by chromosome recombination) are by products of selection for the ability to correct duplication mistakes.
If indeed diploidy was selected for, we can examine situations in which it does not exist, as in human males for the XY chromosome pair.
Review this animation http://www.hhmi.org/biointeractive/evolution-y-chromosome to see what happens when there are not two identical copies and so no perfect recombination match to ensure chromosome duplication integrity. History, as this animation tries to portray, tells us we actually lose chromosomes.
We will follow the lead of most texts and focus instead on theories more tested for the maintenance of sexual reproduction. So in essence we are ignoring how sexual reproduction originated and asking how it is maintained. We should ask this question because of all the disadvantages associated with sexual reproduction. You must find a mate, and often that demands considerable energy and risk. Even if these costs are not figured in, it takes two individuals for every one in an asexual organism to reproduce; so all thing being equal, the reproductive potential of a sexual reproducer is 1/2 that of an asexual reproducer. Another way to look at this is to acknowledge half of the resulting offspring of a sexual reproducer will not directly give birth or lay eggs. This is why the cost of sexual reproduction sometimes has been equated with the cost of producing males.
Go to the following website.
http://www.stanford.edu/group/Urchin/sex.htm
Run the simulation for 10, 20, 50 generations with only 2 young.
Run the simulation again for 10 young.
You should realize the the basic cost of sexual reproduction is staggering and we have yet to add the cost of mating rituals, risk to individuals due to such, etc. This is why evolutionists quickly begin to ask, not how sexual reproduction started but why sexual populations are not quickly driven to extinction by asexual populations. Since we are talking about populations, we are actually asking why one group does not out compete the other. So all theories on the maintenance of sexual reproduction are group selection theories.
Most introductory texts are written by geneticists (Dawkins and colleagues) who focus on genes and favor theory 2. Theory 2 has been shown to work, not in rabbits but in bacteria and protists, where mutation numbers in large populations are very high every generation and because of recombination so are the number of new recombinants with 0 and many harmful mutations in a sexual population. In an asexual breeding population, the mutation numbers are just as high but in the absence of recombination there are very few individual with no mutations and many mutations. Since not as many individuals carry many harmful mutations they are not eliminated and the genetic load, etc. of this population is higher. These asexual populations are deemed less fit than the sexual populations also because of the sexual reproducer's ability to create optimal 0 mutation individuals each generation.
Many evolutionists who work at the organismal level favor theory three, since it involves a ever changing coevolutionary race between the host and parasite and places no requirements on the nature of mutations involved, number of harmful mutations and population size. This has become, despite the favoritism awarded to theory two by text writers , the most accepted theory for the maintenance of sexual reproduction in most multicellular organisms (fish, mites, rabbits, primates, etc.) that do not have populations with the huge numbers found in bacteria and protists. In fact most organismal evolutionists would not agree with those working at the gene level that the maintenance of sex is still largely an unresolved puzzle.
Note that models of theory three only has sexual reproduction winning some of the time. Examine the axes on the graph below and determine the type of world with respect to pathogen/parasite prevalence and influence predicted by these models. This viewpoint has spurred books written for the educated layman entitled "Parasite Rex" and "Parasites rule the world".