The same genetic code can be expressed in different ways. Chemical changes that alter the way the genetic code is expressed can cause two people with the same genetic construct to behave in very different ways.
In 2013, William Rice, an evolutionary geneticist at the University of California, published a paper which linked homosexuality to epigenetics. Despite sharing genetic sequences, in identical twins if one twin is gay, it is more likely that the other twin will be straight than that they will also be gay. This indicates that it isn’t a homosexual gene that determines sexual orientation. However, there is evidence to suggest that homosexuality runs in families. Epigenetics is the only possible explanation that can accommodate the hereditary nature of homosexuality while acknowledging the different sexual orientations of identical twins.
The argument from epigenetics is nothing more than a theory. It has never been supported by empirical data. Just because there is an absence of evidence for other theories does not mean that there is evidence for epigenetics.
[P1] Homosexuality runs in families but identical twins do not have to share the same sexual orientation. [P2] Therefore, the determinant of sexual orientation must be genetic, but not directly linked to genetic code. [P3] Epigenetics is the only explanation that meets these criteria.
Rejecting the premises
[Rejecting P3] There is no evidence for this.