It's not entirely accurate to say that all organisms that reproduce sexually have an even diploid number. While it's true that most sexually reproducing organisms have an even number of chromosomes in their diploid state, there are exceptions.

Here's why most organisms have an even diploid number:

* Meiosis: During sexual reproduction, organisms undergo meiosis, a process that reduces the number of chromosomes in gametes (sperm and egg) by half. This ensures that when a sperm and egg fuse during fertilization, the offspring inherits the correct number of chromosomes, restoring the diploid state.

* Homologous chromosomes: In sexually reproducing organisms, chromosomes exist in pairs called homologous chromosomes. These pairs have the same genes, but they may carry different versions (alleles) of those genes. During meiosis, these homologous chromosomes separate, ensuring that each gamete receives one chromosome from each pair.

Exceptions to the even number rule:

* Some species have an odd number of chromosomes: This is often due to chromosomal rearrangements or mutations that have occurred over time. For example, some species of ferns have an odd number of chromosomes.

* Some individuals within a species may have an odd number of chromosomes: This can occur due to errors during meiosis, resulting in aneuploidy (having an abnormal number of chromosomes). For example, Down syndrome in humans is caused by an extra copy of chromosome 21.

So, while most sexually reproducing organisms have an even diploid number due to the pairing of homologous chromosomes and the process of meiosis, there are exceptions. This highlights the complexity and diversity of life on Earth.