Right... And that girl who has spent all of her teenage years in VR is the foremost author on men, relationships, and romance? If she indeed was referring to the relationship, then it was nothing but kind of adorable bullshit and Asuna humoured her in a suitably benign manner. Thus I'd rather think she was referring to the fact Kirito's interests and choices in life at large might not be too healthy in her educated opinion, as she has seen too much of VR already.
Let's also not forget Yuuki doesn't know Kirito at all. In my opinion he's not even addicted to VR, which is something Yuuki might be judging incorrectly. I doubt Asuna told her anything about the depth of their bond, either, such as how they saved each others' lives. Such things no doubt make a relationship more solid than meeting and dating at random under normal circumstances. They know what really matters.