>>>>How do you know? We do not know the genes associated to sexual orientation (yet). Anyways, a gene defect can occur spontaneously, with no family history. So why would this be different?
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>>>Does not make any sense. You claimed that it is a scientific fact that people are born one way or the other in regards with their sexual orientation, but the gene responsible for this has yet to be discovered? Then it is not a scientific fact, just a theory, right?
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>>First of all, I did claim that there is a firm scientific basis/proof that you are born with it. Scientist are convinced that at least genes play an important role in it. Additionally some prenatal hormones and other environmental circumstances can play a role in it. Since there is a lack of any scientific proof that you develop homosexuality later in life, what other conclusion you want to draw?
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>>If you read the article link I gave earlier, it will become clear.
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>>Second, we know that the problem of heritable cancer is somewhere in the genes, but we still are looking for which ones. Most often it is not a single gene, but combination of genes which makes it much more complicated to pin it down. Can we agree that even if we do not know exactly which genes are responsible for it, hereditary cancer is proven to exist?
>
>But observation, a fundamental task of any scientific endeavor, shows that there is a correlation between parents with cancer and their offspring, as chances increase with family history (as well as on other diseases, such as heart conditions, auto-immune diseases etc.), such thing is not true for homosexuality, AFAIK (and in Wikipedia there are a lot of "hypothesized", "may", "suggested", "could" to convince me).

Read the article again, you will see that statistical observations have been made. Such as the chance of homosexuality is greater when there is in the family line suggesting that it is somewhere in the X-chromosome. Another statistical observation is that with younger siblings have a greater chance of developing homosexuality. Perhaps most convincing is that over 50% of identical twins where at least one was homosexual, the other was as well, while for non-identical twins this was only 22%. This point strongly to genetics, while the 22% for non-identical twins indicate that there must be another component (like hormones).

Anyways, the scientific world generally assumes that ones sexual orientation is mainly determined before birth.

From: http://en.wikipedia.org/wiki/Sexual_orientation

Sexual preference may also suggest a degree of voluntary choice,[9][11][12] whereas the scientific consensus is that sexual orientation is not a choice.[13][14][15]

You of course are free to choose to criticize Wikipedia on the subject.