I can't see it being done without the person having some kinda trauma that made them think that it's okay doing that. I mean... Nina being a girl and Euphie a girl as well.

Not all women who masturbate to thoughts of other women have been molested. I know there are schools of thought to that effect, and I don't know if that's what you were implying, but personally I think it's a little offensive.

Can't Nina's actions stand for themselves without some underlying meaning? Does she need to be redeemed with a dark past? Other characters are allowed to do disgusting acts with little to no provocation, I don't see why everyone keeps looking for some meaning in what Nina has done.

Nina reacts differently to situations than Shirley or Milly would because she's a different person. It's as simple as that.