What If Earth Actually Had Heterosexual Reproduction?

This is an idea I’ve kicked around a while and I don't know if anyone will care, but oh well. As we all know, humans reproduce sexually with the union of a ovum and a sperm. But what if we didn’t? What if one gender fertilised an egg and the other gender could actually *become* the egg, the way it happens in animals like sharks and rays and hagfish and most invertebrates? <br><br>We all know the standard animal mating rituals and the female reproductive tract is pretty complicated in lots of animals. It’s hard to envision how they would be used in this context, but I’ll try to describe how mating would look, given the standard mating behaviors.<br><br>**Mammals**: Probably like standard mammals, the male would mount the female doggy style, but he would deposit the egg, not the sperm. It would slide into the birth canal and the female could fertilise it in the fallopian tube and then nurture it during pregnancy and birth. Lactation and maternal bonds could be unaffected.<br><br>**Birds**: The male bird could probably deliver the egg through his cloaca in a way that might resemble defecation, and the female could then capture the egg in her own cloaca and nourish it with nutrient secretions until it was ready to hatch. <br><br>**Reptiles**: Like birds, the male could probably deliver the egg through his cloaca, and the female could pick it up using her cloaca in a way that resembles mating. Perhaps they could even use their prehensile tails (monitors and skinks have them) to capture the egg in the first place. <br><br>**Fish**: Probably like sharks, hagfish and rays (the only fish with indeterminate sex), males would deliver the egg through their genital papilla, and the female could ingest it through her mouth, where it would then be fertilised in the gut and nourished by her pancreas in a way that resembles the development of some flatworm parthenogenetic larvae. <br><br>**Insects**: Eggs would probably be produced by the male, and the female could pick them up using her ovipositor and fertilise them in her vagina. Perhaps the ovipositor could even become the uterus, and the egg could be fertilised there.<br><br>**Amphibians**: The male could probably deliver the egg, perhaps using a modified bidirectional cloaca, and the female could then capture it in her own cloaca in a way that resembles mating. <br><br>This is just a simple idea for the sake of discussion and curiosity, and I’d love to know what you think about this. Do you have any ideas for how this could look in practice?