What's the purpose of sexual contrast; Why aren't we all hermaphrodites?

Others have hit it on the head, but I'll just re-emphasize.

Hermaphroditism works well among immobile plants (it's pretty much the norm among flowering plants, with a number of interesting exceptions, where there are male and female plants). This is because males and females cannot seek each other out, but are dependent on spreading pollen as widely as possible to as many different flowers of their own species as possible. Often this is dependent on other species as pollinators spreading pollen (male gametes) to the flowers of other individuals (female gametes). So as pollen spreads randomly to other individuals, it cuts the chances of fertilization in half if some of the flowers are male, and some female.

But with animals, the individuals can detect and seek out members of the appropriate sex for mating.

So instead the dominant factor is that it takes energy for an embryo to construct both sets of sex organs. Male organs have to be able to manufacture the tiny sperm cells in large quantities, and to disperse them. Female organs have to be able to manufacture the much larger egg cells in much smaller quantities, to facilitate fertilization, and to protect the developing zygote in its early days (e.g. by producing eggs, or, in the case of mammals, internal gestation.

So male and female organs serve opposite purposes, and are built very different ... so it takes a lot of resources for an embryo to build both. Instead, it is far more efficient if embryos within an animal species specialize in one or the other.

But this gets further emphasized with other adaptations that can change the *appearance* and *behavior* of the two sexes to further enhance their specialized roles. In the simplest form, males try to mate with as many females as possible, and females try to maximize survival of offspring. But then males and females can learn to select each other for purposes of *mutual* protection of the offspring. It gets complicated ... but the point is that these are all options available to animals through behavior, that are not available to plants.

So that is why male-female specialization is so much more successful in animal species.

With hermaphrodites genes get degraded quickly. The way to balance this is to have millions of offspring and have more simpler genetic code. The exchange of genetic material in sexed species offers a stability that drives the forces that shaped the male female polymorphism, and also drove for larger more complex lifeforms.

Different biological roles. It is proven to be more successful. Males have short term goals of spreading as much seed as possible. Females have long term goals of having as many survive as possible. Those different biological goals tend to shape different forms.

i might have loaned a number of my previous adult males out for strictly sexual applications... a number of them have been too large to be so egocentric as to no longer proportion. All women individuals could desire to be entitled to savour that form of delight. hahaha