In Laura's case the reason she wash herself is because of shame, she loves the MC deeply, yet she let another man defile her, and thus she feels dirty, and feeling dirty leads to her washing her complete and especialy the places he touched her. But also woman compare men with eachother and if Matt is on all parts better then her partner is they will start liking it more what Matt does then what her partner does. But to analyze Laura complete you need to know, who wrote her, was it a man or woman, then a woman or man is also molded from birth to present, that all counts in how a character will be what they are in the present, but i'm not a shrink, I can only vision her personality from what I see and my knowledge.
There is a parameter for affection, on love path that obvious should always kept high to 100%, but on other paths that is falling down and you see her love for Matt grow while for her mc fails especialy the NTR path. Another thing is some women get horny from dominant men, and Laura is obvious a submissive woman, that would also be a reason she likes what Matt does to her.