One of the most compelling arguments for me would be the following on male siblings:
If one was homosexual then the chance of their sibling being homosexual was:
Fifty two percent for their identical twin, who shared 100% of the same genes. This result was essentially identical to the separated identical twin studies described below. It shows that if one identical male twin is gay, then the other twin will probably also be gay -- whether raised in the same family or raised by different families.
Twenty-two percent for their non-identical twin; they share half of the same genes
About 10% for adopted or non-twin brothers; they share none to half of the same genes.
Now, let me pose the same question to you that I've posed to others on this thread:
Why would a young adult that hasn't been exposed to the bad behavior, that hasn't been abused, that's had the best environment, that hasn't had any of the behavioral reasons to be gay, be gay anyway?