Animals have much more complex genomes. They are more at risk from mutations, and they cannot reproduce as quickly. There is more incentive to "get it right", if you will. With asexual reproduction, selection occurs at the level of the genome, meaning you are as good as your "worst" gene. You can't get rid of harmful mutations, and it seems like (complex) species that get stuck in asexual reproduction go extinct.
With sexual reproduction, selection can occur at the level of the gene, allowing beneficial mutations or recombinations to spread through a population AND correcting (many) harmful mutations.
To achieve the same without sex, mutations would would have both break up the co-adapted genes without breaking their functions and also find a good alternative mutation. With sex, it does not matter if co-adapted genes are broken up, because they are forced to evolve to deal with it. (This is kind of similar to the concept of learning to learn in neural network research.) Also, if a parent gene is unfit, it is possible that the child can escape this mutation by receiving that part of the genome from the other parent instead by a 50% chance.
That sex introduces lots of variation is undisputed, however, and the mechanism of sexual selection accelerates the process over that of the random drift in asexually reproducing species.
The original question is... assuming that we have mandatory sexual reproduction, why do we have a 1 to 1 ratio?
Only reptile, amphibious, fishes and it seems that very primitive mammals also can do this. Our system to get rid of the excedent is to make males sillier, much more prone to take life-threatening risks than females.
1 [Those aren't our only options, we have the so called supermales (one X, two Y), and superfemales (XXX) but they are not common and some combinations ("1.5 X") are sterile]
That's not a sufficient explanation, as there could be a different ratio during the sperm cell generation process or selection during insemination or from the egg cells themselves. Y and X sperm cells could easily be handled differently at any point of the whole process.
While sexual reproduction occurs through spawning, where genetically diverse offspring travel far and wide on currents to possibly different habitats and niches.
The idea here is that asexual reproduction is a benefit to the species during relatively stable conditions, while sexual reproduction works best to populate new areas or recover from population disasters.