Why wont he take us getting married seriously?

Here is the answer to your question, taken right from your own post: "he is pretty much getting everything out of a marriage: kids, sex, a "wifey" at home, ect, without a commitment." That's the reason he won't marry you. You have given him everything you had to give without getting back the thing that you wanted - and now you're silly enough to allow him to keep wasting your time with promises that he never intends to keep.