I can't say whether it's a "sin", because that seems like an entirely subjective question. If you think it's a sin, then I guess it is, since you're the one who decides what a sin is at the end of the day.
I can say masturbation is psychologically, emotionally and physically healthy behavior for both men and women. It reduces stress and anxiety, aids sleep, enhances self esteem, lowers the risk of prostate cancer and heart disease, helps balance sexual relationships where one partner desires sex more than the other, and when done as a couple reduces the risk of STDs (if that's all you're doing) and helps couples map each other's pleasure centres.
I think women should be wary of partners who disapprove of masturbation, because it's very common for women to be unable to climax from vaginal intercourse alone. A relationship with a partner who disapproves of either of you manually stimulating your clitoris may turn out to be a relationship where you never experience sexual satisfaction. Ever. It's not worth the risk, IMO.