I /think/ they're saying that if sexual reproduction came first, you have a pretty basic problem: where's the second individual that the first sexually reproducing entity mates with?
Often times sexual organs are multipurpose. It's likely a genetic mutation that allows for sexual reproduction, as with all structural advances. I might end up with a functioning probiscus near my gamete sack and if I end up bumping into my fellow eukaryotes, that's just how it goes.