Hello clueless. Welcome to the board. You are at a great place with a lot of people who understand what you are going through. I know as a woman in a SSM you can feel so alone and freakish kind of. Because the typical stereo type we always hear about is men want sex all the time.
I can totally hear the pain in your words. I can say I know how you feel. I to have a husband with a low sex drive. I to have a husband where I have seen evidence that he masturbates. It is heart breaking I know. But one thing you need to know or come to terms with is that it's not you. It's him. I don't know maybe our men are wired differently. Maybe they have hang ups that we aren't aware of. But don't make yourself sick over this. You have to stay strong and healthy for you and for your children.
One thing I can say is in a small sense you are lucky that you are not married yet. So you do have a choice. It's much easier to leave when there is not the string attached that you said wedding vows. My advice would be to walk away if you know deep down that this will never change. I would try and have a heart to heart with him and let him know in no uncertain terms that you are unhappy with your intimate life. Tell him this is something you would consider leaving over.