Marriage, sex and disability ... what should I do?

Don't do it. I understand that you have needs but what your contemplating on doing will put a strain on your marriage & a heavy burden on your sick wife. I'm in the same boat as you & your wife is. Suffering from an (degenerate) autoimmune disease. Although we aren't at a stage where sex in painful for me, it has somewhat effected our sex life as far as the physical requirements. What you need to do is come up when new ideas on how to position your wife that makes her comfortable. Use pillows, neck rolls, toys etc. if you have to. Also there are other ways to gain sexual gratification without the actual penetration that she can perform on you & vice verse. But having sex with your friend of 25 yrs. is not the answer to your problem. If anything the only thing you'll accomplish is more heartache for you & your wife. And for what? For a moment of sexual gratification? It's not worth it if you love your wife.