Let's say you have a certain source of information - a book for example - and you decide to make a copy of it. Let's assume that there is no digital or mechanical technology available so you have to copy it on your own. Inevitably you will make some mistakes. Now what if, after you are finished, another person comes along and he creates a copy of your copy, then yet another person creates a copy of the copy of the copy? Imagine the results after - let's say - 10 million iterations! It is very likely that the final copy wont have too much to do with the original source. But if we make some rules it is possible to get more favourable results. Let's say we forbid to make a copy by one source alone, and we establish the following rule: one may create a copy if, and only if she/he finds two distinct sources which are word by word compatible. Probably it will take much longer to reach the same number of iterations, but it is sure that the final copy will be way more accurate than in the previous case.
This is a very simplified description of sexual reproduction. The purpose of sexual reproduction is to preserve the initial information, to preserve a certain state and layout of genes and genetic instructions. Sexual reproduction doesn't leave too much room for divergences. Biologists make a huge mistake when they extrapolate changes that they observe on bacterial populations to macro evolutionary levels, not only because the differences between reproduction rates are astronomical (if we compare bacteria to vertebrates), but because they completely ignore that sexual reproduction is not as forgiving towards mutations and divergences as fission.
On the Darwinian account sexual reproduction is a self-defeating mechanism. On wikipedia there is the following statement on this matter: “The evolution of sexual reproduction is a major puzzle.”7 The presence of genders sets up very strict limits for evolution. If an organism that reproduce this way is born with certain mutations - it's no matter if those changes make it the fittest, strongest, smartest animal ever lived -, and its DNA diverges from the rest of its species, it wont be able to reproduce. This is huge evolutionary drawback. Sexual reproduction also cause tremendous vulnerability against extinction. If an environmental catastrophe or a disease decimates a population and only a few members survive - the most resistant and fittest ones -, it is possible that the survivors consist only of members of a certain gender, and thus the whole species go extinct. This situational and circumstantial nature of sexual reproduction cause a huge evolutionary disadvantage. You always have to find a member of the opposite gender to reproduce. The genetic diversity and possibility of gene exchange/combination can not justify these tremendous drawbacks, since bacteria have much simpler and more effective ways to achieve the same results. Horizontal gene transfer and bacterial conjugation make them capable of exchanging genetic information between each others. If evolution is true, why hasn't this system advanced forward, why do we have a way more complicated and more circumstantial system?