A Deepness of Thought
Gonochoristic is the opposite of hermaphroditic. In gonochoristic reproduction, individuals are either male or female but not both. About 3-4% of plant species are gonochoristic. The rest of plant species are hermaphriditic. A few animal species are hermaphraditic, mostly mollusks. The vast majority of animal species are gonochoristic. Hermaphroditic reproduction is much more efficient than gonochoristic reproduction because all individuals can reproduce. Hermaphroditic reproduction can be obligatory out breed to achieve the similar genetic diversity as gonochoristic reproduction.
So what is it about a species' mobility that makes gonochoristic reproduction such an advantage?
Insects are gonochoristic and they are the epitome of mobile. They are highly adaptable, with short life spans and large numbers of offspring per generation. The variety of insects is staggering. Butterflys and dragonflys migrate with the seasons hundreds of miles over several generations. Butterfly metamorph to completely different body plans. Yet in all their adaptation, no insect species is hermaphroditic.
Gonochoristic reproduction is faster adapting than hermaphriditic reproduction. By having two distinct sexes, a species can divide its risks. Males can take on more risks than females. This allows the species to reap the benefits of the risking with half the exposure. Lets take for example the amount of fur a mammal grows to protect itself from the cold. Suppose the females could grow and loose fur with the temperature change while males must keep a fixed amount of fur. Male fur follows a normal distribution with some males having a lot of fur and some having little but they do not grow more or less of it. Now suppose the climate radically cools down to the point that half the males freeze to death while most of the females grow more fur and survive. The next generation will have much more fur since they inherit this trait from the surviving males. Also, the population is little impacted since most of the females survive. Now imagine this same scenario with hermaphroditic reproduction. Almost all the individual grow more fur and survive. The hermaphroditic species is very slow to adapt at the genetic level and can not compete with the gonochoristic species.
This could explain things like why men have weaker immune systems than women. It is adaptive to the survival of the species. By some of the men dying of disease, the next generation is more resistant. A species with a stronger male immune system could not compete with a species with a weaker male immune system.
Gonochoristic reproduction is faster adapting while hermaphroditic reproduction is more efficient. So one possibility is that mobility makes the faster adaptation of gonochoristic reproduction more significant than the efficiency of hermaphroditic reproduction.