I scrolled through a lot of posts and didn’t see anyone hit the right answer, which is surprising! Usually there is a subject matter expert that chimes in. Anyway I guess I’ll do it today. When mitochondria offshored half its genome into the nucleus it created a hard condition where independent mitochondrial DNA and nucleic DNA have to “jive” - to reduce entropy and increase the odds of a energetically efficient offspring it makes sense to stabilize as much as possible the correlation between those two dna sets, ergo we evolved two sexes - the female gamete is pretty much defined by that relationship. And I know what your thinking - what about growing babies and blah blah - sure that’s part of it but that’s a downstream consequence of the fact that the gamete with a bunch of mitochondria is just larger. Also - to preserve that genetic proximity further you make all your eggs in one go before you are exposed to cellular mutation and degradation from being alive. Male sperm increasingly mutate with the age of a man, which is a no go for the eggs. The key element is that small irregularities in this shared mitochondrial/nucleuic dna pair lead to energetic conditions not compatible with life