Sexual Orientation- A sexual inclination
Most commonly used to describe the gender of one's sexual inclination, but that would be only a fraction of one's sexual inclinations.
This thread is first a comprehensive definition of sexual orientation and than a opportunity to discuss the impact of sexual orientation on people's lives.

If your not familiar with Alfred Kinsey's gender orientation scale, he claims that you can chart one's gender orientation from 1 to 7. 1 being completely heterosexual and 7 being completely homosexual. 4 is equally attracted to men and women. Where do the tranny lovers fit in? What about pediphiles?

Before Kinsey there was Magnus Hirshfeld (Germany late 1800's). His idea, which inspired Kinsey, was that you can rate one's sexual orientation on scales of 1 to 10. Each sexual inclination would be rated seperately. One's heterosexual inclination would be rated on this 10-point scale, along side one's homosexual inclination. These scores can be described as such:
1no inclination
3could do with or without
6prefered
9almost completely focused
You can see from this method how an individual may be heterosexual 9 and homosexual 9, or a strong bisexual. The range of sexual orientation is vast within these two catagories alone, but that's not all.

You can also scale any other sexual inclination on this 10-point scale.
The Pansexualities:
Heterosexual
Homosexual
Asexual
Sapiosexual, or attraction to the mind.
Autosexuality, attraction to one's self.
Transgender orientation, etc, etc.
Now you can see how one may be Heterosexual 9, homosexual 2, and yet still have a sexual attraction to Transexuals 6(The chicks with dicks phenomenon that so many "straight" men identify with).
The Paraphilias:
Exhibitionism, Voyeurism, Sado-masocism, scoptophilia(porn), galactophilia(lactation), pediphilia, etc, etc, etc. There are hundreds of these specific sexual inclinations. Any one individual could conceivably scale there own inclination in regards to each but that would be a waste of time. Most people would rate themselves a 1 on all but a couple.
Promiscuousness
This is one last extremely important aspect to one's sexual orientation. Is one inclined to have one sexual partner or more than one? This can be scaled on one 10-point Promiscuousness scale. 1 meaning no inlination toward promiscuity (or monogamous), 9 being almost exclusively focused on having multiple partners.

What does sexual orientation have to do with you? Do you understand yours? Do you know the sexual orientation of your SO? If you find that your SO's sexual orientation is not compatable to your own, can it be reconciled? Do sexual orientations change overtime or can they? Can you successfully change, stifle, or ignore the sexual orientation of you or your SO?