To briefly answer your question, yes, that hurdle has been crossed. Even bacteria swap DNA from time to time, so there are very few examples of truly asexual organisms out there anyway.
But there is a bigger question. Even if this answer was still unknown, why wouldn't evolution still be considered a theory? If the answer were still unknown, then this would be an area that could POTENTIALLY falsify evolution. However, not knowing does not automatically falsify a theory. A theory is an explanation for observations, not ALL observations. At the same time, a theory can not be falsified by any observations. Not knowing how sexual selection evolved, or could have evolved, does not falsify evolution. Therefore, even without this knowledge, evolution would still be considered a theory.