My issue with the terminology is that the available terms for "more than girlfriend, but not (yet) engaged/married" are so...clinical. "Partner" sounds like it could be a business partner, and "Significant Other" is even worse (And the abbreviation "SO" can cause confusion; at least once, someone thought I was using the word "so" with EXTRA EMPHASIS).
I agree that "girlfriend" for long-term committed relationships is problematic, but so are the alternatives. We really need a better term before we start formalizing this particular relationship stage.
This is why I don't use "partner". I've had business partners and in conversations people constantly thought I was referring to a spouse.
Also, partner can refer to a married or unmarried spouse, so it doesn't specify anything.
Lastly, if I refer to my DH as my partner, people tend to immediately assume I'm gay and referring to a woman, which isn't accurate either.
There's nothing wrong with using "partner", sometimes you want some ambiguity, there's no need for everyone to know whether or not you are gay or straight, married or unmarried, etc. "Partner" is great for not having to specify the details if you don't want to.
But if you want to be specify, then it doesn't work very well.
I'm not saying I like "girlfriend" or "boyfriend", I just don't think they're wrong or disrespectful to use when someone wants to specify: romantic partner, unmarried