https://sci-hub.se/10.1126/science.324_1254 https://sci-hub.se/10.1126/science.324_1254 In short, sexual reproduction is very old (> 1 billion years ago) and f

https://sci-hub.se/10.1126/science.324_1254 https://sci-hub.se/10.1126/science.324_1254 In short, sexual reproduction is very old (> 1 billion years ago) and first appeared in an ancestor of all eukaryotic cells and long predates multicellular life. So, all of multicellular life evolved with the haploid/diploid molecular machinery already available and different branches of life have built vastly different strategies to use it. You'd be better served IMHO to start at DNA replication and build up rather than starting at sex organs and working backward.