1. There is no definitive gene responsible for homosexuality.
The presence or lack of a single gene says nothing about the overall extent to which a trait is influenced by heredity. There is also “no definitive gene responsible for” skin color, height, handedness or many other innate human characteristics because these traits, like sexual orientation, are influenced by multiple genes acting in concert with one another. That doesn’t make the traits non-genetic, it just makes them complex.
In fact, twin and family studies indicate that genes are the major known factor responsible for individual sexual orientation in both males and females, accounting for 25 to 50 percent of the overall variation and 50 to 100 percent of the ascribable variation; the sources of the remaining variation are unknown, but are most likely to be biological and stochastic factors since they are not affected by rearing environment or social surroundings....
2. Homosexuality is not a disease, but merely an abnormal behavior which may be learned through experiences in life.
There is no scientific evidence that homosexual orientation is a learned behavior any more than is heterosexual orientation. ...
3. In every society, there is a small number of people with homosexuality tendencies.
This is correct, but it points to precisely the opposite overall conclusion of the report. Cross-cultural universality is an indication that a trait has origins that are shared by all humans, most likely genetic. Traits that differ from one culture to the next are more likely to be learned.
4. Homosexuality can be influenced by environmental factors, such as culture, religion and peer pressure among others.
Although sexual activity can clearly be influenced by environmental factors, including laws, the underlying orientation is immutable.