What would happen if diploid organisms reproduce by sexual reproduction without first producing sex cells through meiosis?

If no meiosis takes place chromosome number will be doubled disturbing the genetic make up of the organism.

Explanation:

If no meiosis takes place then the resulting zygote will be of double the number of chromosomes. The genetic makeup will change entirely. The process of meiosis halves the number of chromosomes in haploid cell and maintain the serenity of organism.

The process of meiosis due to crossing over, genetic variation occurs which let the offspring receive favourable alleles to survive in new conditions.

In diploid organisms first the gametes are formed, then fertilization occurs and the diploid cell is formed which gives rise to the organism having diploid cells.