Don't get me started on the "Available" thing as I have done (and still do to some extent) the poly thing. Maybe it's a cultural thing but here we don't say we are "dating" after we've been together for 10 years but simply aren't married (yet). Dating and being in a relationship has two very distinct meanings here.
Edit: Not to mention to more complex relationship types outside of your primary. You can have additional lovers, be dating, have a secondary live in partner, be in a triad, and of course the various BDSM dynamics that can be outside of your primary relationship. (E.g.: Vanilla with primary, Domme of others, sub with secondary.)