What is the ruling of Islamic law regarding a wife's refusal to engage in marital intimacy, and is she considered sinful? What is the ruling on the husband resorting to masturbation to relieve his desires in this situation?

A wife must allow her husband to enjoy her whenever he desires. If she refuses without a valid excuse, she is defiant (nashiz) and disobedient to her Lord, and the angels curse her. Even if she is on the oven, she must come to him if he calls her. She does not fulfill the right of her Lord until she fulfills the right of her husband. If she does not feel desire, she must still obey him to protect him [from sin], and she should seek medical reasons [for her lack of desire]. It is not permissible for the husband to masturbate. The husband should advise her and treat her, otherwise, the solution is to marry another woman while keeping her and treating her kindly.