There is the so-called "Uncle" theory which can explain how the homosexual "gene" is passed on and maintained within populations.
Assume a man is homosexual and does not sire any children during his life; therefore, none of his genetic information will be passed on to a new generation. However, he may have heterosexual brother and sisters. They will be approximately 50% genetically identical to the homosexual man and some will probably carry the homosexual gene even if they are not themselves gay. Any children that they have will have 25% genetic identity with their gay Uncle and may again pass on the homosexual trait to their offspring. Although the gay uncle does not therefore contribute directly to the next generation's genes, he does so indirectly through his nieces and nephews. Moreover, if he is active in helping to raise and nurture his nephews and nieces and ensure their survival he increases the chances of his gay gene being propagated in future generations. Some theorists believe that gay Uncles may be gentler and perhaps more feminine than straight Uncles which is why they make better Uncles and better carers/protectors for their nieces and nephews.
The part an Uncle plays in raising his nieces and nephews may be reduced in today's modern world but in primative hunter gatherer societies (and even 100 years ago when we all lived as closer family units) an Uncle may have played a much greater role in protecting his brother or sister's children until they reached adulthood and had children of their own.
if it was a gene, bisexuality would come into question.
....
and there could also be the possibility that it is a choice- most people do not become "gay" until later in their life, maybe from failure with women, and it may not be a gene. I mean, is being straight a gene? It will always be present, your sexual orientation. Maybe it was curiosity and they liked it.