Personally, whenever I hear it or read it in a romantic context I just assume the person using it is gay and talking about his or her gay lover.
Occasionally though it turns out s/he is not gay, however, and instead is using the word 'partner' anyway in some kind of weird unisex way to be more inclusive or something.
It's basically a 'conformist MSM acolyte' identifier, because this is the label coming out of feminism and the gay camp in academia and their advocates in the mainstream media, that everyone is supposed to use to refer to any kind of lover or spouse as 'partner', in order to blur all distinctions between sex and sexual orientation (the idea is to push 'tolerance' by in essence confusing everyone so nobody has a concrete idea what anyone else is talking about, and instead there is just this vague abstract notion of "some kind of romantic affiliation, of some indeterminate degree of seriousness, with some unknown individual of unidentified sex"). Most people using it aren't actually activists; they are just MSM disciples conforming with the language they consume there.
Ultimately it strips both the sex and the degree of seriousness from the label, making it a much vaguer and lower information label, and the person using it an inferior communicator. Seems to be the way a lot of language is trending nowadays though: vaguer, lower information, less descriptive, less effective communication.
Confucius would be displeased.
Chase