Why do women consider thier past and sexual expeience as a non issue.?

I have been with a number of women who (without knowing my own past)...consider the number of men and the types of sexual encounters they have had as a non issue...And yet. If they take into account "other" girls who have been with say 30 or more men...or me who have been with 20 or so women...they consider these people whores or sluts. Why do almost ALL women view thier OWN past as though they are some angel and that no-one has a right to question thier activity yet they will STILL pass judgement on others...I am bitter I admit it, but its not just bitterness. Almost ALL the girls I ask think the same way. So why is it? Please help...Why is it that every single girl I know thinks she is a "good girl" ...whats the psychology behind this "not me" mentality. What IS a "good girl" number 3, 5, 10, 20....?? Or is it really that they DO feel guilty and some I know have even called it thier "whoring-years" yet for some reason its "ok" and beyond question...I suppose the real question is ...is this all just an immature attempt at avoiding that emotional pit of "it is me"...or is there some other process I am missing.