Cooeymama, are you still around? First, I want to give my opinion on something. Your boyfriend has threatened to "go out and get it from someone else" several times. He's not the kind of man to marry. You may not be able to see it now, but he has probably done you a favor by not proposing marriage. Maybe he does love sex more than he loves you. You need to get to the bottom of this before even thinking of marrying him and that won't happen if you just give in to sex when you don't feel like it. How about counseling?
The counseling would not be to get him to quit asking for sex or for you to learn to give it to him when you're exhausted. It would be for learning communications skills, talking about respecting another person's needs (like your need for sleep!), and working out the problems. Will he invest that energy into your relationship?
Just curious--does he work full time as well? Does he help with your child? Is he cleaning house, cooking, etc?
I have to say this--just because it's the only thing you argue about, it doesn't mean that giving him more sex will make everything peaceful. If he has no emotional investment in the relationship, he won't bother arguing about other issues. If he is there for the sex (and perhaps food, laundry service, etc), he may only bother to argue about that. See what I mean? Do you think he is emotionally connected to you? Is he respectful of your time and energy? How about your feelings? Does he care?
You have your hands so full! a 1-1/2 year old, a full time job AND housework, etc--not to mention a boyfriend who threatens to cheat--something has to go. What do you think it should be??
How old are you?