Why do people deny evolution?

We would probably need to go back to the Precambrian for that. We could start by asking how Pikaia reproduced.

Possible ancestor[edit]
Much debate on whether Pikaia is a vertebrate ancestor, its worm-like appearance notwithstanding, exists in scientific circles. It looks like a worm that has been flattened sideways. The fossils compressed within the Burgess Shale show chordate features such as traces of an elongate notochord, dorsal nerve cord, and blocks of muscles (myotomes) down either side of the body – all critical features for the evolution of the vertebrates.

Bear in mind that the nature of sexual reproduction really has everything to do with the exchange of genetic information and not with the development of organs for assisting the process.

And as Parados has pointed out, all of the important drivers behind sexual evolution happened at cellular levels long before mammals arose.

Meiosis Listeni/maɪˈoʊsᵻs/ is a specialized type of cell division that reduces the chromosome number by half. This process occurs in all sexually reproducing single-celled and multicellular eukaryotes,

And as Parados has pointed out, all of the important drivers behind sexual evolution happened at cellular levels long before mammals arose.

I can't find any information on how they reproduced so I assume it is asexually. So, are we assuming that this animal evolved into opposite sex versions as we move up the evolutionary ladder to complex heterosexual vertebrates?

I guess I am interested in learning how evolution altered the genetic information through natural selection of random mutations, so that an asexual animal could evolve into a heterosexual animal over several generations.

I would especially like to know how the male version evolved alongside the female.

Males and Females are not separate species evolving along side each other. They are the same species with differing expressions of a genetic characteristic (in our case it's a single chromosome).

Males and Females are not separate species evolving along side each other. They are the same species with differing expressions of a genetic characteristic (in our case it's a single chromosome).

Examples in animals[edit]
There are examples of parthenogenesis in the hammerhead shark[21] and the blacktip shark.[22] In both cases, the sharks had reached sexual maturity in captivity in the absence of males, and in both cases the offspring were shown to be genetically identical to the mothers.
Reptiles use the ZW sex-determination system, which produces either males (with ZZ sex chromosomes) or females (with ZW or WW sex chromosomes). Until 2010, it was thought that the ZW chromosome system used by reptiles was incapable of producing viable WW offspring, but a (ZW) female boa constrictor was discovered to have produced viable female offspring with WW chromosomes.[23] The female boa could have chosen any number of male partners (and had successfully in the past) but on these occasions she reproduced asexually, creating 22 female babies with WW sex-chromosomes.
Polyembryony is a widespread form of asexual reproduction in animals, whereby the fertilized egg or a later stage of embryonic development splits to form genetically identical clones. Within animals, this phenomenon has been best studied in the parasitic Hymenoptera. In the 9-banded armadillos, this process is obligatory and usually gives rise to genetically identical quadruplets. In other mammals, monozygotic twinning has no apparent genetic basis, though its occurrence is common. There are at least 10 million identical human twins and triplets in the world today.
Bdelloid rotifers reproduce exclusively asexually, and all individuals in the class Bdelloidea are females. Asexuality evolved in these animals millions of years ago and has persisted since. There is evidence to suggest that asexual reproduction has allowed the animals to evolve new proteins through the Meselson effect that have allowed them to survive better in periods of dehydration.[24]
Molecular evidence strongly suggest that several species of the stick insect genus Timema have used only asexual (parthenogenetic) reproduction for millions of years, the longest period known for any insect.[25]
In the grass thrips genus Aptinothrips there have been several transitions to asexuality, likely due to different causes

I am assuming that asexual reproducing animals can not reproduce heterosexually until they develop opposite sex organs through evolution. Is that a wrong assumption?

I am assuming that asexual reproducing animals can not reproduce heterosexually until they develop opposite sex organs through evolution. Is that a wrong assumption?

I am assuming

Some plants and animals reproduce bot sexually and asexually. There ought to be some clues in that, somewhere.

I believe there are several examples of simple life forms

Those simple life forms are basically the same sex with the same sexual organs.

How did they evolve the complex opposite sex organs, and the complex behaviors to get from simple to complex heterosexual reproduction?

Did a worm or some other simple animal evolve to an animal with partially developed opposite sex organs that after generations evolved to complete opposite sex organs?