Omnisexuality (sometimes called Pansexuality) is a term used to describe undiscriminating choice for sexual and\or romantic partners. Most people are familiar with the term "bisexuality", which includes both "homo" and "hetero" partners for sexual and\or romantic affairs. Pansexuality extends beyond bisexuality, to include the sexes that are usually referred to as "others", including -but not limited to- transsexuals (people who undergone a sex-change operation) and intersexes (people who don't fall in either male or female profiles). Omnisexuality even expands beyond pansexuality to include sexuality with non-humans.
Personally, I like to identify myself as omnisexual, which roughly means: "I'd fuck anything that has a hole I can put my dick in!" Okay, that was an overshoot of a simplification, but guess you got the idea.
Some might think that extending sexuality to non-humans would include "zoosexuality". Technically yes... But since I consider consent to be an important precondition to sex, which pretty much excludes animals of the list. The reason I extend non-human species on the list is because [in my futuristic fantasy] I would be open to have sex with intelligent alien species - [Okay, guess Star Trek messed me up in this part]. So, I consider myself open to relationships with any species that I can have a mind-to-mind chat with.
Sensual Side of Sexuality:
Sexuality is a primary carnal drive. Sex is generally considered as a source for pleasurable sensations. Since, the primary reason for engaging in sexual activity is pleasure, then it seems unreasonable to discriminate pleasure according to its source. That's to say, if two things provide exactly the same sensual experiences, then those experiences are equal regardless of their source.
Consider this simplest act of getting a blow job, that is, using the mouth for sexual stimulation. Since a male's and female's mouths provide roughly equivalent experiences, then if you close your eyes and someone (either male or female) gives you a blow job, the sensation is the same, and the pleasure is the same!
I remember a friend once telling me about a film where the camera starts rolling with really nice feet, moving up to show nice shaved tall legs, moving further up to show a really nice ass... At that point, as that friend describes started wanking his dick... The camera kept moving up until it was revealed that the person shown was a guy - not some hot chick. As that friend describes, it totally blew the moment! He then goes on to explain that he thought about the incident for a while and later on realized that, since that nice ass was indistinguishable from a hot chick's ass, that the marginal fact that its a guy's ass shouldn't stop him from enjoying that nice ass!! And I totally agree with him, it's exactly the point I am making.
Emotional Side of Sexuality:
In my personal understanding, sexuality represents acceptance. When you can accept someone then you would not mind getting naked around them, and allowing the exchange of pleasure. In this understanding, it seems to me, the more people you accept, the wider the sexual orientation would expand.
Mutual acceptance seems that most emotionally gratifying product of sexual activity.
In that light, I consider my hypothetical acceptance of the idea of having sex with an alien, as a symbolic acceptance of what might seem weired or "alien" to us as earthlings.
Conscious Choice of Sexual Orientation:
Opposed to the popular view that sexual orientation is determined during early stages and is immutable, I disagree and think that
It is known that most people have bisexual urges, but people later on focus their sexual orientation towards one of the sexes (usually the opposite sex). I don't really see why some people insist on limiting their sexual orientations to "hetero only" or "homo only"... It doesn't make perfect sense to me. Why make only 50% of people as potential sources of pleasure, when it is possible to have 100% of people as potential sources of pleasure?!
Obviously, our social constructs increase the tendency of people to pick one of the genders as potential sexual partners. These constructs usually defined stereotyped views of males, females, and the relationships between people of different genders. For example, the socially constructed views about the importance of sexuality between males and females drive most people to the opposite sex, while this social pressure might negatively affect others and drive them away from the opposite sex towards the same sex. In my opinion, in many cases [but not all] sexual orientation is an extension of gender stereotypes AND stereotyped gender roles
Edit: The last section was modified to resolve some ambious interpretations