This allowed a size asymmetry to get started, and the asymmetry helped, so it became more extreme as time passed. Today, a male creature may give off millions of sperm, and an ostrich bundles an immense amount of food resources with each egg cell. The sperm do all the dangerous travelling. Eggs are guarded and protected. So, these species lost the idea of two equal haploid cells merging.
Some trees have both male flowers and female flowers.
Some fish can change from female to male. However, humans use a simple genetic scheme whereby some offspring grow only male sex cells, and some grow only female sex cells. The chemical cues which control this difference also cue some other bodily specializations. In the last billion years, the bodily specializations have become increasingly intricate. Many sea creatures just expel clouds or ropes of eggs into the ocean.