The argument from epigenetics
The same genetic code can be expressed differently depending on chemical changes that take place before birth.
Homosexual and heterosexuals do not have the same neurological structure.
In 1991, Simon LaVay, a British-American neuroscientist, carried out postmortems on women, men assumed to be homosexual, and men assumed to be heterosexual. He found that in heterosexual men four cell groups known as the INAH3 which regulate male-typical sexual behaviour. Women and homosexual men had much smaller INAH3 cell groups. This research was collaborated by the results of a 2008 study when the Stockholm Brain group found that lesbian women and straight men shared larger INAH3 groupings.
This does not mean that we are born gay. Our brain is not fully developed at birth. Therefore, the discovery of neurological differences between gay and straight people does not mean that we are born gay. LaVey himself even admitted in 1994 that just because homosexuals and heterosexuals have different neurological differences, it does not necessarily indicate that they were born this way. He also admitted that the brains he studied had died from AIDS. He could not rule out that the neurological discrepancies were not caused by the disease.  Additionally, neuroscience is an area in which we still know very little. The correlation between regions of the brain and behavioural traits has not been proven beyond doubt.
Enter the framing of the argument here ...
[P1] Homosexuals and heterosexuals have different neurological structures. [P2] Neurological structures are determined by factors that occur before we are born. [P3] Therefore, people are born gay.
[Rejecting P2] Neurological structure does not emerge before birth.
Enter more information about the argument here ...