Very interesting question janikon. I don't think there really is a black and white answer for this question. There are claims that it is genetic and others believe it is upbringing.
It is believed by some that the gay attribute that some individuals live with spawns from an inherited gene. The same concept relating to such diseases as Diabetes or certain mental illnesses, which have been proven to be hereditary.
But, once again, homosexuality and lesbianism have not yet been proven either way to be inherited.
On the upbringing side of things, it is understandable to consider the idea that environmental conditioning may have such effects. Basing this on the fact that humans are generally very adaptable, especially during our upbringing stage of life. But how adaptable?...Can we be influenced through environment to have a totally opposite sexual preference in life?..I really don't believe that question can be answered.