Your wife is the victim of a serious trauma and is showing many of the signs of sexually oriented post traumatic stress. This reaction, often seen in abuse victims, is a way for them to distance themselves from the feelings and impact of genuine sexual intimacy.
For your wife, right now, the only way she can handle sex is to make it immaterial, distant and cold. Masturbation reduces sex to an activity, a thing to do to release an urge. Telling you that she does not mind if you go out on your own and satisfy your sexual needs with another is a defense mechanism. In no way does she, at a real level, mean this; this is the trauma and fear talking. If you actually did, the odds that she would leave you in the future are very good. This is only a lie that she is perpetuating to help her deal with the real problem, a fear that the closeness of real sex would bring.
She is afraid, most likely petrified, of abandonment and what that would mean if she got close to you, as would happen with "real" sex. Keeping you at arms length sexually is her way of coping with that fear. It works to an extent, Yet, it is a terrible way to deal with the pain she has experienced, and it resolves nothing.
A typical couples counselor usually is not enough in these cases. She has emotional trauma, and a trauma trained therapist is needed. Techniques such as EMDR, (a non medically oriented verbally based therapy for trauma victims) and other skill sets are used to assist those affected. The person does not need to have high motivation to do so, just a willingness to try. Your family doctor should be able to help you with a referral to a counselor who uses these techniques.
For you, I would highly suggest reading Overcoming Childhood Sexual Trauma: A Guide to Breaking Through the Wall of Fear for Practitioners and Survivors bySheri Oz and Sarah-Jane Ogiers. This resource will help you work through things with your wife and to understand deeply what she is doing, why, and how you can support her through the therapy process.
This can be resolved, but the first step is to see a trauma trained therapist. This reaction is able to be stopped, and she is showing the classic signs of the problem, but with the right help she can be whole again. Steven