Morality is in the brain

Liberals are very moralistic. They believe social policy ought to reflect their values. This includes homosexual marriage, abortion on demand, and veganism.

Let's consider this from the standpoint of neuropsychology. In general, liberals are physicalists. They believe the brain is the source of the mind. 

According to neuropsychology, the frontal lobe is the source of consciousness, emotion, decision-making, and problem solving. The parietal lobe is the source of language. The temporal lobe is the source of  long-term memory, while the hippocampus is the source of short-term memory.

I'm oversimplifying. For instance, the temporal lobe is another source of language and emotion. 

Admittedly, this is a fairly crude description. My argument doesn't depend on localizing every function in an airtight compartment of the brain. There's a certain amount of redundancy and plasticity. 

We can debate where to draw the lines. But in general, neuropsychology does say different parts of the brain generate different aspects of human personality. 

In that respect, the human brain is analogous to an android brain. If you were a cyberneticist, you'd add or subtract specific cognitive functions by adding or removing the corresponding hardware. LIkewise, a neurosurgeon can subtract specific cognitive functions by removing the corresponding lobe. 

Suppose a liberal thinks homosexuals have a right to marry each other. From a neuropsychological standpoint, that just means a part of his brain tells him that homosexuals have a right to marry each other. If a neurosurgeon removed that part of his brain, he'd instantly cease to think homosexuals have a right to marry each other. Conversely, if it was medically feasible, a neurosurgeon could make him think homosexuals have a right to marry each other by adding that missing part to his brain.

On this view, morality is reducible to adding or removing relevant parts of the brain. Like adding or removing a chip from an android brain. 

Suppose E.T.s have frontal lobes that are differently configured than ours. Their frontal lobes don't tell them that homosexuals have a right to marry each other. Evolution programmed their frontal lobes with different moral instincts than we have. Their temporal lobes tell them that homosexuals are food. 

Assuming, for the sake of argument, that neuropsychology is true, what makes it a fact that homosexuals have a right to marry each other? If their values come and go depending on the presence or absence of the corresponding brain part, isn't that an arbitrary combination? Something extrinsic to right and wrong? Like programming a terminator android? Replacing one part with a different part changes morality.