One big problem I see with sexual reproduction ever happening would be that two of the same species in the same location would have to "evolve" to have the ability to reproduce sexually. Then those two would have to meet, fall in love (ha ha), and reproduce. Then their "children" must stay in the same general area and repeat the process. This seems to be extremely unlikely to me. It seems far more likely that anything that developed the ability to reproduce sexually would find that trait to be negative in the fight for survival, and it would die out.
Of course, if anyone thought that evolution actually worked that way it would, indeed, be a problem. But it doesn't so the problem you see doesn't arise.
In general, evolution
can't move in big jumps. What you seem to hint at in the above is "instant speciation" (saltation). This does happen but only under very special circumstances.
Sexual reproduction simply will not arise that way.
We see existing organisms that have various stages of part way between our dipolar sexual reproduction and asexual reproduction. Obviously if various stages exist now and work just fine then organisms could have
gradually moved through those stages.
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