Article: Male sexual orientation influenced by genes, study shows

A study of gay men in the US has found fresh evidence that male sexual orientation is influenced by genes. Scientists tested the DNA of 400 gay men and found that genes on at least two chromosomes affected whether a man was gay or straight. 

A region of the X chromosome called Xq28 had some impact on men's sexual behaviour – though scientists have no idea which of the many genes in the region are involved, nor how many lie elsewhere in the genome. 

Another stretch of DNA on chromosome 8 also played a role in male sexual orientation – though again the precise mechanism is unclear.

The cause of homosexuality in males is a fraught subject. There are, basically, three possible causes: free choice, genetic determination, and envirnmental factors.

Most male homosexuals say that, looking back, they knew they were homosexuals from an early age based on what and who attracted them. And they reject the idea that they chose a homosexual orientation.

It is obvious from previous studies on twins, going back to the 1980's, that there is a genetic component to male homosexuality, although there is not a 1:1 correlation with genes and homosexuality as there is with genes for eye color or (most) hair colors.  It is much lower.

Wikipedia: Biometric modeling revealed that, in men, genetic effects explained .34–.39 of the variance [of sexual orientation], the shared environment .00, and the individual-specific environment .61–.66 of the variance. 

In other words, while genetics plays a factor causing male homosexuality, its effect is half the size of enviornmental factors, whatever they may be.

The flawed thinking behind a genetic test for sexual orientation is clear from studies of twins, which show that the identical twin of a gay man, who carries an exact replica of his brother's DNA, is more likely to be straight than gay. 

A very clear example of the larger effect of environmental factors.