Are gay people born that way? Maybe so.
Sergey Gavrilets, a NIMBioS researcher and an author on the paper that outlines the new theory of homosexuality, published in The Quarterly Review of Biology says, "It's not genetics. It's not DNA. It's not pieces of DNA. It's epigenetics. The hypothesis we put forward is based on epigenetic marks".
To be specific, the new theory suggests that homosexuality is caused by epigenetic marks, or "epi-marks," related to sensitivity to hormones in the womb. These are compounds that sit on DNA and regulate how active, or inactive certain genes are, and also control when during development these genes are most prolific. Gavrilets and his colleagues believe that gene expression may regulate how a fetus responds to testosterone, the all-important male sex hormone. They further argue that epi-marks may help to buffer a female fetus from high levels of testosterone by suppressing receptors that respond to testosterone, for example, (thus ensuring normal fetal development even in the presence of a lot of testosterone) or to buffer a male fetus from low levels of testosterone by upregulating receptors that bind to the hormone (ensuring normal fetal development even in the absence of high levels of testosterone). Normally, these epi-marks are erased after they are activated, but if those marks are passed down to the next generation, the same epi-marks that protected a man in utero may cause oversensitivity to testosterone among his daughters, and the epi-marks that protected a woman in utero may lead to undersensitivity to testosterone among her sons.