From my basic understanding:
Attraction is surprisingly biological. Your bodies goal, no matter your personal goal is, is to have strong and healthy children. When searching for a mate, your body is looking for someone that compliments you genetically and immune system-ly (could not think of a word). Everyone has their own signature smells and pheromones that they produce to accomplish this. Males and females, of course, produce different pheromones and smells due their sex hormones. In homosexuals, their bodies like smells and pheromones from the same sex (for the most part). Just like everything else, there are exceptions to the rules. It is possible to make changes to the brain through the epigenome and nurture. How big the changes and which makes the changes, epigenome or nuture, is up for the debate. For all we know, the epigenome is the tool nurture uses to make changes.
Personally, I don't think the changes can be that big if it's just nurture and there's no traumatic event. People have tried to nurture homosexuals out of homosexuality for centuries. You'd think it work the other way too.
Mind you, this is "find a mate" attraction and not "what you're willing to have sexual relations with". The porn industry is proof enough that people will 'do' anything and anyone.
Also, just because we don't have a genetic link now, doesn't mean we won't. There are thousands of genes to figure out and we only just begun. Not to mention it's much more important to find genes of genetic diseases than it is to find the "Gay Gene(s)".
You might call me an idiot inwardly while reading my question, but what about the sexual attractions of people who can't smell?...