Hm. I guess it's good because: other terms are gender-specific. Even if you personally don't care for being gender-neutral, you could agree that at the very least, there are certain contexts where it is A Good Thing for people to avoid indicating the gender of their partner, e.g. conversatin' with conservative family friends and being able to omit rather than lie. The thing is, this only works if the convention of using 'partner' is adopted by people in straight relationships as well, because otherwise, it'd be obvious that anyone not specifying 'girl-' or 'boyfriend' was queer.
Also idk man, if you think about it 'girlfriend' and 'boyfriend' are pretty strange terms too?? I have a relationship with this person that is specifically romantic in nature, so I'm just going to prefix the term 'friend' with 'girl' or 'boy,' words that connotes youthfulness and immaturity. In other words 'girl'/'boy' are terms that designates the sexual aspect
and I mean, yeah this is probably related to calling yer beau 'baby'. Anyways from this BS nonsense I just typed it seems that perhaps the terms 'girlfriend' and 'boyfriend' reinforce the notion that without marriage, intimate relations are somehow juvenile or immature as they are still in the domain of young people, or conversely that only young people do not marry.
But ultimately I am probably inclined to justify the use of 'partner' because I do happen to hear it used pretty often and thus find it familiar
In conclusion language is strange!!