Playersexual to me is one of these two things:
1)Seems to go against their stated orientation, but ONLY for youuuuu.
2)Is specifically interested in you and only you, regardless of your sex or anything else about you. For example, if Liara was human and behaved the same way, I'd have to go with her being playersexual. But she's an asari, so I would take it as more bi/pan/omni/whateversheis.
For DA2, I'd call them playersexual but with a bisexual tone. They're there for your specific story. However, if we consider their range of options and how their script lines come across, it would imply that they can be attracted to both sexes.
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Anyway, I do think there's a small difference between pansexual and omnisexual.
Pansexuals don't really think or feel in terms of gender. It's all one to them, and they focus on the individual person and/or experience.
Omnisexuals may think or feel in terms of gender, but it just doesn't matter to them. They'll move between orientation 'modes' as they come. One time they may be more into males, but another time they may fall in love with a female and stick with her.
They're very similar, but the main difference to me is that Pansexuals are largely divorced from concepts of gender/sex/orientation when they decide to do something, while omnisexuals still factor it in.
Omnisexuals may do anything.
Pansexuals may do anyone.
They're close enough that one is usually splitting hairs when trying to use them in different ways though. Pretty much synonyms, but again, only pretty much.
I'm up for any corrections here, though.