IIRC, if you start from equal sized gametes, cheaters can win by providing less, which requires the others to provide more. In the end you get stable sperm and egg gametes where one provides 100% and the other packs only the energy to swim. Hence n=1 decays into n=2.

A lot of other sex characteristics arise as consequences, including the relative huge numbers of male sperm to female eggs, the fact that the sperm usually swims and the egg doesn't, the female becoming the site of internal fertilization, the female carrying and feeding the pregnancy, the female looking after live born young, and the consequent genetic pickiness of females versus male fire-and-forget.

As for the stem cell thing, one of the kinds of cells they can be made into is the kind that become gametes. In principle though not yet in practice you should be able to make either kind from either sex - they have already made female sperms in mice. Hence sexual reproduction will be available to anyone with anyone, or solo.