This page answers the commons questions, Why do men like lesbians? and Why are so many women bisexual? (see footnote 1), elegantly demonstrating that these two obversations can be easily explained by evolutionary theory, and do not contradict it as many insist (see footnote 2). Note that, at the time of writing, this theory has not been published in a peer-reviewed journal and should therefore be regarded as a hypothesis.
Note to feminists: This theory is merely an explanation — it does not justify men’s behaviour any more than a theory on why men like violence would justify brutality — so please don’t send angry e-mails.
Alpha-male behaviour
The fossil record suggest that early humans lived in moderately-sized family groups. In modern primates (with the exception of Bonobos, discussed below) all such groups are dominated by an alpha-male, so it is reasonable to assume this behaviour is inherited from a common ancestor, and applied to early humans as well.
An alpha-male rises to that position by deposing the previous alpha-male, and maintains it through the brutal oppression of any challengers. As a result, the alpha-male is the most powerful individual in the group, and can mate with any female he chooses, regardless of her consent.
It is in a female’s interest to mate with the alpha-male, as this will result in her children inheriting the genes that helped the alpha male to rise to power. Fortunately for the lower-ranking males, it is also in a female’s interest to mate with them as well, as this ensures a wider genetic range in her offspring. As a result, females and lower-ranking males will often mate covertly.
It is not in the interest of the alpha-male to mate with females who have recently been fertilised by other males. One way to avoid this is for the alpha-male to prefer homosexual females, who will occasionally arise through genetic mutations or recombination, and can easily be identified by their behaviour towards other females. Homosexual females will not be interested in the males, and so the alpha-male will quickly hear if a lower-ranking male attempts to mate with a homosexual female. The alpha-male can force himself upon these females, leading to his female offspring being more likely to be homosexual.
Deception by bisexuality
Because the homosexual females only mate with the alpha-male, their offpsring to not have as much genetic variation as females who could mate with the alpha-male and the lower-ranking males. If females could deceive the alpha-male into believing they were homosexual, they would out-compete the true homosexual females.
There are two ways this can happen. Heterosexual females could display sexual behaviour towards each other in order to make the alpha-male believe they were homosexual. Alternatively, females could become bisexual, in which the case the sexual behaviour towards other females would not be an imitation.
Anonymous psychological surveys of women suggest that the latter option is more common than the former — many surveys suggest that most women have at some point had sexual feelings for other women.
The latter option could have evolved more easily, as women already have the genes that lead to attraction to females. The Y-chromosome carries very little information, and acts more like a switch, turning on or off genes in other chromosomes. As a result, men and women have most of the genes required by the opposite sex. This is easily demonstrated by the fact the the Y-chromosome does not turn off the genes that lead to men having nipples, as they do not cause men a disadvantage even though they serve no purpose.
Bonobos
Bonobos were mentioned above as the exception to the alpha-male rule. Bonobo society is instead dominated by the females, who famously use sexual intercourse as a means to settle disputes. Bonobo females will frequently have sexual intercourse not just with males but also with the other females.
The theory above can easily explain how this situation could have arisen from an alpha-male dominated society, if the observation is made that male attraction to lesbianism can be used by women to influence men.
Footnotes
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In this report, published by the Centers For Desease Control And Prevention, National Center For Health Services, US Department Of Health & Human Services, 11.2% of 61,561 female respondents reported having engaged in sexual activity with a same-sex partner.
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“At the annual meeting of the American Association for the Advancement of Science, leading researchers and theorists in the evolution of sexual behavior will gather to present the growing evidence that Darwin's idea of sexual selection requires sweeping revisions.”
Further reading
For more how evolution has affected human sexuality, see Jared Diamond’s book Why Is Sex Fun?: The Evolution of Human Sexuality.