Since sexual preference is determined through genetics, no, you cannot change it without gene therapy. And gene therapy doesn't really exist yet, in a full form.
I'm not sure if I can find it but the gist of it (a research article on fruit flies) shows that by selectively breeding the flies for a certain gene, they can increase the homosexuality rate. You might try searching for "gay flies" or "homosexual flies". I'll try it now.
I'm not saying it's influence is 100%, as I believe that environment can play a role. For example, intelligence is estimated to be 90% genetics and 10% environment. There is no real proof of this gene in humans, but most likely the gene is there, as genetics shape the person we are.
There may be a preference gene but children develop in huge bounds mostly before 3 years old with this tappering off as they get older.
A child tends to live in a world of first impressions and they are exploring pleasure as much as anything. Humans definitly connect at a higher level than just body language.
One person can effect hormonal, chemical balance in another by mentaly connecting.
I would think that the people or a person in the childs life were connecting and stimulating certain chemistry into action and making it pleasurable.