I have to disagree. Sexual orientation IS a major part of a person whether they are gay, straight, bi etc. It is not however, a definitive explanation for someone's personality or existance as a whole but the fact still remains that alot of what we do as people, as sexual beings, in life, is driven by our sexual orientation.
My turn to partially disagree. The degree that sexual orientation matters to a person varies from person to person. This is because the sex drive itself varies -- and varies over time. It is certainly a bigger thing when a person is a teenager and in their twenties. For some people that never goes away. I know of one guy who couldn't keep it in his pants and left his wife just before the birth of their third child because he'd found a younger woman to be with. Clearly, for that guy his sexuality was more important to him than his family.
How often does my sexual orientation mean anything to me? Honestly not that much. Most of the time I'm at work I'm not thinking of it at all (okay -- all the younger girls and guys can laugh, but there aren't many hot guys where I work so it's not something that is constantly drawn to my attention). Equally, when I'm with my boys it doesn't matter at all -- I'm a father first, being gay comes a long way behind that fact. The main time it matter is when I'm writing, or thinking about a story.
Just my observations....