Right, but why? The energy-cost of making these sexual organs themselves, WITH NO REASONABLE GUARANTEE OF WORKING (and even likelihood of failing), is the question. Asexual reproduction is "the safe bet" and, since evolution is a process, there's not any reason to implement it: that is, evolution cannot 'see' that added effort/complexity* now yields some benefit in the future. (*Something that is 'optional' won't be used in the first rounds of evolution and should fall victim to degenerating ["vestigalization"?], and an entire system [sexual reproduction] is of no use if it doesn't fully work.)
Many animals are simultaneous hermaphrodites (worms, snails, clams), and some fish are sequentially hermaphroditic, and change gender during their lives.
Except self-fertilizers it's irrelevant to the discussion: in order for sexual reproduction to work it has to be multiple members having sex. -- Thus energy is wasted on sexual-organs, from a evolutionary-standpoint, if there are not other sexed organisms of that species to mate with: at that point it actually becomes true that sexual organs should be the ones selected against.
Sexual reproduction may be a very early development. One theory is that it developed as a method of repairing damaged DNA through an exchange with neighboring cells. The benefits of sexual reproduction are clear, and if a population retained that characteristic, in certain environments (for example where damage was likely), it could outperform competitors.
Sexual reproduction may be a very early development. One theory is that it developed as a method of repairing damaged DNA through an exchange with neighboring cells. The benefits of sexual reproduction are clear, and if a population retained that characteristic, in certain environments (for example where damage was likely), it could outperform competitors.