Incest should be legal because homosexuality is

[Brian37]

Evolution in all life works better with diversity. And chemically speaking genetically related family actually produce pheromones that are a chemical scent mammals give off noting relationships. And just like other mammals, like elephants and lions, they too seek outside the family. Incest while not physically impossible is not advantageous to spreading genes.

There are even with adults moral and ethical reasons outside evolution we tend to avoid it. But old books of mythology do nothing at all to explain human behavior. Trying to explain human behavior with an old book of mythology, is like trying to explain astrophysics with astrology.

Evolutionary biology, human psychology are better ways of saying why most humans avoid incest.

Can you extend on this? Given that you have two related adults, engaged in consensual intimacy, with no desire to procreate.

Not arguing the existence of incest. It happens. All I was arguing is a general scientific fact that most of the time our species avoids it. 

I would say also that even if it is claimed to be consensual, it is far more likely that one or both of the adults were products of child molestation even if they consent as adults.

Like I said, even with other mammals, evolution did not evolve to favor incest. "Consent" is most certainly an adult issue. But even then, that does not mean those consenting adults are basing their relationship on anything mentally healthy.

Mammals including humans, literally give off chemical smells to denote the difference between family and non family. 

While it is not impossible for two adults not abused to have sex within the family, it is far more likely that even between consenting adults, that incest is a product of abuse by a parent or someone the victim is familiar with when they were younger.