Academics at the University of Padua, Italy, claim that bisexual men may have inherited their attraction to men through "hyper-heterosexual" female family members.
Dr Andrea Camperio Ciani and colleagues showed that the female relatives of homosexual men tend to have more children, which they believe suggests that genes on the X chromosome are responsible.
The team now believes that the same is true for bisexuality.
The researchers carried out a survey of 239 men, asking about their families and sexual experiences.
The results showed that gay and bisexual men's female relatives on the maternal side had more children than those of straight men.
Dr Camperio Ciani stressed that this does not prove the existence of a "gay gene", but that an unidentified genetic factor promotes sexual attraction to men in both men and women.
This in turn would influence a woman's sexual attitude (but not increase her fertility), making her likely to have more children.
California Neuroscientist Dr Simon LeVay describes this genetic factor as "hyper-heterosexuality", and claims that it would help pass homosexuality on through the generations.
Dr Camperio Ciani and colleagues say that the genetic factor appearing in both bisexual and gay men supports the heory that sexuality is determined by a mixture of genes and experience.
Dr Camperio Ciani told The New Scientist: 'We understand that the genetic component has to interact with something to produce different phenotypes.
"Genetics is not determining the sexual orientation, it's only influencing it."
Dr Camperio Ciani studys sexual behaviour and sexual strategies in primates and humans, and victims of sexual crimes.