Hello First of all there is no co-relation between masturbation and stretch marks on stomach. Those might be because of other causes like sudden weight gain or weight loss ,, strenuous gym workout ,or physical exercise like football game or other outdoor activities. It's very difficult to remove stretch marks . you will need clinical procedures like dermaroller or lasers . You can visit near by skin doc n take his/her opinion. As of now you can apply, Retin A 0.05% cream on that area at night time only . Keep the area well moisturized with Cetaphil moisturizing lotion . Cap eposoft E once a day after food for 1 month (orally) Thanks