Straight women are much more likely to get themselves knocked up than gay women. So, in terms of evolution, they would seem to have a better chance of passing on their genes, while at the same time it would seem that the genes that make women gay would quickly vanish from the gene pool. This raises the question, why are there gay women?
Lesbianism is indeed at least 25 percent genetic, as determined by a 2011 study of twins conducted in the United Kingdom. The study found that identical twin sisters (who share 100 percent of their DNA) are more likely to both be lesbians than are fraternal twin sisters (who share just 50 percent), proving that, all other environmental factors being equal, genes matter. While scientists have a theory for how male homosexuality propagates from one generation to the next, no one has yet produced a viable explanation for how the genes that promote lesbianism might do the same. [Why Are There Gay Men?]
A female's sexual orientation also appears to be partly influenced by her level of exposure to the male sex hormone androgen when she is in the womb. Greater hormone exposure correlates with more gender nonconformity early in her life (as a child, she may be called a "tomboy"), as well as a same-sex orientation later on. A study by Dutch psychologists published in the March issue of the Journal of Sexual Medicine reported that 10 to 12 percent of male and female children who feel discomfort with their gender go on to identify as gay or lesbian as adults. Meanwhile, just 1 or 2 percent of children who are comfortable with their gender identity turn out to be gay or lesbian.
Adding to the confusion about what causes lesbianism is the slipperiness of female sexuality itself. Unlike men, who are usually sexually oriented solely toward men or women, and whose sexuality is essentially fixed from puberty on, a decade of research by the University of Utah psychologist Lisa Diamond and others demonstrates that women have greater "erotic plasticity." Their sexual orientation can be shaped by cultural influences, altered by positive or negative experiences and intensified by feelings of love or attachment. Women are far more likely than men to "report remarkably late and abrupt onset of same-sex sexuality, often after heterosexual marriage," Diamond wrote in January in the Archives of Sexual Behavior.
Their sexual fluidity may emerge from the fact that, across the board, women are sexually aroused by images of both men and women (while men are typically only aroused by members of their preferred sex).
Therefore, the question "why are there gay women?" may be better worded as "why is female sexuality so fluid?" Plenty of women exist at both extremes of the straight-gay spectrum, but it is the formation of this slippery spectrum itself that most needs explaining. Evolutionary biologists have yet to determine what survival or reproductive benefit women's "erotic plasticity" confers.