Well, sex had existed for many hundreds of millions of years previous to this.
Even before multicellular organisms existed, single celled eukcaryotes (protists, more complex than bacteria, but still single cells) had been having sex, as they still do today. Basically it's just an alternation of generations. Haploid (half the genetic compliment) protists fuse to create the diploid (full genetic compliment) generation, which then creates the haploid cells, and so on.
The origin of sperm simply meant that the gametes, the sex cells, were now dimorphic, having different male and female forms. Before that they were the same. Initially this was probably a very minor innovation, and only grew to have great significance retrospectively as many other associated changes and innovations were added on.
Your replies were an excellent addition to the thread.
Thanks for making the case so succinctly!