Since it kinda takes one to know one, I think very many of the questions of women from men here stem from curiosity about women's views on penis size, or the man's insecurity about his own size.
I think the thinking behind the question you address here is along the lines of this:
First (universal) assumption: Women are on LPSG because of the LP part of that abbreviation. They, i.e. women belonging here, are, deep-down, consciously or not, size-queens of some sort on LPSG seeking to get a LP fix.
Second assumption: If a woman is on here without her steady male partner knowing, she must be dissatisfied with his size, hence she's on LPSG to partially satisfy her (psychological, visual) craving for LP, and
Third assumption: Is secretly on LPSG because she doesn't want to hurt her guy's feelings by being honest with him that she isn't satisfied with his size and honest about her true feelings about size.
Fourth assumption: Maybe my wife, GF, ex-wife, ex-GF, woman I'm interested in, etc. is, has been, or would be a member here if she knew about it, because if she isn't satisfied with my size, she's going to seek to satisfy her craving for bigger somehow, someway.
Finding out how many women are on here without their male partner knowing is merely data to try and figure out what the women in that guy's life--past, present, future--think about size and might be doing without his knowledge.
That's my translation of the question.
I think the thesis that it's fishing to learn what women's relationship statuses are is likely false.
Finding a way to tie a guy's question to his curiosity about women & size/his size insecurity more often than not probably leads to the best explanation here on LPSG. Not a brilliant or original insight, but I think it applies to the question addressed by dolfette.