Susan,
You have a very big problem. He is impotent but does not want to take Viagra. If sex is very important to you which I know it is, you need to see a urologist who will suggest other treatment of choice for impotence. If your husband will refuse to remedy the situation, I think you should file for a divorce.
On the other hand, if you can live with your husband without having sex, stay in the marriage. I know some people will advise you to use a vibrator. For me a vibrator is not a good substitute for the real thing.
If I was in your situation, I would file a divorce because he refuses to do something about the problem.
You need to weigh things, the ultimate decision is yours and you have to be comfortable with it.
Take good care.