This is similar to the argument they make about the eyeball, that half an eyeball is useless, so how could eyes have evolved? It's a straw man argument, saying evolution teaches eyes evolved as they are in humans and all at once then calling it impossible, so then everyone nods their head and think how much smarter they are than silly scientists.
The reality is there are organisms that live in the ocean with a very crude version of an eye ball that simply sees the difference between light and dark. This gives them an advantage over organisms with no sight as they can see that a predator is approaching. From that point on anything that improved sight was an advantage to survival and would get passed on.
Sexual reproduction is no different. Early versions of sexual reproduction was very simple, but gave an advantage because mixing DNA was an advantage over non sexual reproduction. It gradually become more complex as organisms became more complex.