The fraternal birth effect shows people are born gay

Question: Are people born gay?
Position1 of 4›: Yes, people are born gay
Argument‹3 of 6›: The fraternal birth effect shows people are born gay

The more older brothers you have, the higher the likelihood of you being attracted to members of the same sex.

The argument

For every older brother someone has, they have a 33% greater chance of being gay. However, this does not apply to adopted siblings. The linear correlation between fraternal births and homosexuality indicates that there are genetic or hormonal impacts that dictate a child’s sexual orientation.

Context

Studies indicate that the more older brothers you have, the more likely you are to be born gay.

Premises

[P1]For every older biological brother a person has, their chance of being gay rises by about 33%, an effect that does not hold for adopted siblings. [P2] Because the correlation tracks biological rather than adopted brothers, it points to genetic or hormonal influences on sexual orientation rather than environment. [C] Therefore, because the fraternal birth effect reflects prenatal biology, people are born gay.

Counter-arguments

If this argument were true, then as family sizes decrease, the number of gay people should also drop. It would also suggest that in places like China, which adopted a one-child policy, the number of homosexuals would drastically decrease. This has not happened. The number of gay people in society has remained stable despite family sizes decreasing significantly.

Rejecting the premises

[Rejecting P4] If this was the case then as family sizes decrease, so would the number of gay people.