It's only 'common' with the lesbian couples who adopt a style of appearence - any other lesbian couple look like two friends.
Therefore, I'd say that far more lesbian couples do not choose to affirm their sexuality by means of outward apearence, and it's actually the minority that do so.
I understand that you are not offering criticism, but i would reiterate my point - it is simply a choice of style, in the same way as a puink, goth, emo, etc., and i don;t believe there is any more to it than that.
However, this is just my own viewpoint, and there are a number of gay women on the AB who can come in and offer a far more valid viewpoint than I.