With the traditional theories, as eukaryotes are held to descend from asexual bacteria, sex has to be invented in some way or another after these organisms had got a nucleus. Nobody has presented a believable theory for how sex could have originated from non-sex. There are however theories on maintenance of sex. These theories explain why sex has not been lost. The reason there is a need for such theories is that loss of sex seems to have some benefits, and there are species that practice sex only very occasionally.
With the Organelle Escape Theory there is no need for any invention of sex. If eukaryotes were the original, then cell fusion was in place very early, and it is quite natural that there would be different variants that would be maintained. Reduction of the number of sexes would however be natural, and the most probable end situation is reduction to two sexes.