We've had this discussion before. Left to their own devices, yuri fans would tag every single image with "yuri"; to preserve the focus of that tag, we had to give them something else to tag things with, and so "2girls" was born. It's meant to be treated as "I think it's yuri but it's not so obvious" by yuri maniacs, and as noise by everyone else.
Search the forum for the previous discussions before posting, or you'll end up wasting everyone's time. I'm looking at you, Fencedude.