Why, if there is a tradition of incest in the early parts of the Old Testament, is incest considered wrong today? Is it because we are now aware of genetic diseases? (I've put this in Christianity, but really it covers all Abrahamic religions)

It is a very beautiful thing called a 'contradiction'.

While this is an old question, you haven't gotten a clear answer. Here goes:
Your first reading is correct. Genesis 12 first tells of Abram calling his wife, Sarai, his sister. He does so again in chapter 20 (note that by this time their names have changed to Abraham and Sarah). Verse 12 shows that they are half siblings.
Earlier, Genesis 5:4 tells us that Adam has "sons and daughters" who would have intermarried to produce the human race. (Genesis 3:20 indicates that there were no other humans outside this original family.)
To answer your question: There isn't so much a tradition of incest as a reporting of the situation early in human history. At that time, there were not the genetic diseases associated with close marriage that came to exist later. Because Man was created perfect, with everlasting life in view, he started with no genetic impurity. With passing time, the defects due to sin multiplied.
Consider that a copy (such as a document) can't be better than the original. A copy of a copy of a copy will pick up every imperfection of what came before and possibly add to them.
Thus, only about 400 years after the time of Abraham, God gave strict laws limiting closeness of relatives who could marry.--Leviticus 18:2-18 (If you read 'lay bare nakedness' or something similar, it's a Hebrew way of saying 'have sexual relations'.)
Whether they understood all the reasons for it or not, God's people received protection from the One who did understand. When they obeyed these laws they prospered and suffered when they rebelled.
While some societies still practice endogamy (marriage within the family), to most peoples it is repulsive and taboo.
So, put another way, early humans sometimes married close relatives, but they did not commit incest in the modern sense. By God's rules, it became a crime about 25 centuries after He created Man.

While this is an old question, you haven't gotten a clear answer. Here goes:
Your first reading is correct. Genesis 12 first tells of Abram calling his wife, Sarai, his sister. He does so again in chapter 20 (note that by this time their names have changed to Abraham and Sarah). Verse 12 shows that they are half siblings.
Earlier, Genesis 5:4 tells us that Adam has "sons and daughters" who would have intermarried to produce the human race. (Genesis 3:20 indicates that there were no other humans outside this original family.)
To answer your question: There isn't so much a tradition of incest as a reporting of the situation early in human history. At that time, there were not the genetic diseases associated with close marriage that came to exist later. Because Man was created perfect, with everlasting life in view, he started with no genetic impurity. With passing time, the defects due to sin multiplied.
Consider that a copy (such as a document) can't be better than the original. A copy of a copy of a copy will pick up every imperfection of what came before and possibly add to them.
Thus, only about 400 years after the time of Abraham, God gave strict laws limiting closeness of relatives who could marry.--Leviticus 18:2-18 (If you read 'lay bare nakedness' or something similar, it's a Hebrew way of saying 'have sexual relations'.)
Whether they understood all the reasons for it or not, God's people received protection from the One who did understand. When they obeyed these laws they prospered and suffered when they rebelled.
While some societies still practice endogamy (marriage within the family), to most peoples it is repulsive and taboo.
So, put another way, early humans sometimes married close relatives, but they did not commit incest in the modern sense. By God's rules, it became a crime about 25 centuries after He created Man.

Where in the early parts of the old testament do they endorse incest. Can you give an example?

YOur kids will come out with retardation...Is it worth it...I have been reading so much about this...Look at the Amish and the Hasidic Jews, they are inbred...

wow

i think that the egyptian's and the following generation's considered incest was good, in order to preserve the DNA that existed between the family members as a way to not invade their familial DNA....that is kinda what introduced hemophilia........i am soooo not making this up........Brian......

i am catholic......Matthew 22:39.......yes....Cain and Able had incest with their sisters......Cain killed Able.......look at the egyptians and the era of people of royalty wanting to keep it in the family....Queen Victoria.....Alexandra and Czar Nicholas....that is how we have hemophilia today........take care...Brian......

It's becasue of the possibility for diseases and a massive cultural revolution.