Finally someone got the science worked out!
It appears being gay is linked to a number of genes which, when present in the right combination, lead to the expression of homosexuality. It appears the most powerful genes, GMBT2 and AFDJ9, when both present increase the probability of homosexual orientation to around 72%. The probability increases if there are other associated genes also expressed in combination with one or both of these, though it seems there is quite a range of associated genes that can act in combination with various relative 'strengths' (ie the more genes you have - the more likely you're going to be gay).
Environmental exposures, including chemical and hormonal influences can cause both genetic mutations and alterations in the genetic expression of genes already present, influencing the probability of homosexual orientation.
Following from this there looms the possibility of antenatal genetic screening tests, estimating the 'probability of orientation' but with environmental influences ongoing in-utero it seems these would be loosely indicative and not be as accurate as testing after birth. Post-birth testing may then become available with genetic manipulations to 'cure' homosexual orientation, and while no such treatment is available, its possibility is expected to cause significant social and ethical issues.
Full article soon to be published in the journal 'Nature'.
You heard it here first.