M has not treated you well but you continue to hope that things will change. After talking with M - where you are probably withholding sexual feelings / desire for him - you need to release that sexual tension / desire and then turn to J to do so. You are correct to be concerned about J since he may develop more than merely sexual feelings for you when you are not prepared or desirous of falling in love with him.

My suggestion is to break off contact with both while still remaining friends. End all contact with M. I understand this may be hard since you still have deep feelings for him. But, the fact remains, he likes to play the field and is not ready to settle down. M cannot give you the permanent relationship you are seeking because you have different values. I suggest you seek counseling to get over him so that you can go out and meet someone who would be a more compatible and emotionally healthier mate for you.