Does masturbation make me thin or not?

Masturbation is not the cause of your problem, unless you are jerking off so much that you don't have time to eat. There are tons of guys who have been masturbating for decades (like me) and are of normal weight or overweight. You don't say if you have always been this thin, or if you have lost weight recently, but you should have your weight checked by your doctor and get an estimate of your daily calorie intake to make sure you are eating enough. If you are healthy, have adequate calorie intake, and do some resistance training, you should be able to add some lean body mass