I recognize that it would do little to help species that merely dump gametes and leave because specialization of one reproductive system would likely do the job better and both genders equally contribute under that type of system. Clearly there are good reasons, since gender (or asexual repr) is the norm, but I need more/better evidence.
You have presented one example of hermaphroditism and used that as evidence that all species should be hermaphrodites.
Therefore at higher population densities, when mating opportunities are not rare, the gonochoric individuals will have a higher fitness because they have more energy.
Hermaphrodite hook up site
I honestly don't have any clue as to how daunting (or simple) that is, but I'd imagine that the sophistication of the systems would play a pretty key role.
Makes me wonder how much this has been researched in true hermaprhodites.
But most organisms are not internally fertilized mammals with wildly complicated systems of internalized embryonic care.
Most species lay a pile of eggs that a male squirts sperm on or squirt eggs while the male squirt sperm and then they hope for the best.
I don't have a reference, but I was thinking all multicellular life and I'd have to imagine that when you factor in plants, that external fertilization is relatively the norm (as is hermaphrodism (dioecy) for the plants).