I come from being a 40 year old gay man myself, so I hope that counts as experience. Ha! Ha!
I thank you for your question to which I would love to reply. First I am not a scientist so I can not explain the genetic reason for it other than the chromosome/gene facts that have been published. Just Google gay genes.
I would first point out that there have been many examples that just because gay people are gay, do not assume that all gay people do not desire to pass along their "genetic information". I point out the following article:
http://wiki.answers.com/Q/How_do_gay_peopl... My husband and I have been together for over 12 years now. We have actually often thought about adopting. However we believe that it is more responsible to take care of a child that is orphaned somewhere, rather than just passing along our genetic information. We don't speak for all LGBT people by any stretch of the means, this is just how we feel. We have 7 gay couples, as close friends and they all have adopted babies, and it is amazing to see those children that once had no hope, are growing up in loving happy families.
Regarding when did evolution deviate, I pulled the following from Google.
"The first recorded evidence of homosexuality is found in Mesopotamia, circa 3000 B.C. where artifacts have been discovered depicting same sex couples. Alexander the Great is also recorded as having had a same sex relationship. Both predate the Romans and even later Greek culture -- in the case of Mesopotamia -- by a great deal. I'm not sure about earlier recorded evidence.
There are other types of recorded evidence roughly the same age however, for example, in the ancient necropolis of Saqqara in Egypt there is a very old tomb for a male/male couple who oversaw the Pharaoh's manicurists. That tomb is from the old kingdom and dates to between 2600 and 3000 BC also.
Anything older would be in the East (China, India and the Indrus valley) and archaeological work of the type that would turn up such evidence is much rarer there. The oldest known writing on the topic from the Chinese Empire is from "Master Yen's Spring and Autumn Annals" and refers to a ruler in 547. There certainly were older records, but whether any survive is a different question, and whether, if they do, they will be recovered and available as sources in the West during our lifetime, even more so."
http://answers.yahoo.com/question/index?qi... Message me if you would like to talk further.
On a totally unrelated side note, I have to close with the following. LGBT high school bullying has hit a high this week with 6 LGBT people taking their own lives because of in school anti-gay bullying, hate speech, and invasion of privacy. Please support and repost:
http://www.thetrevorproject.org/float-nav....