Notice how long this thread is? If it weren't an issue this would have died 24 pages ago.
My wife and have been married 25 years and still go 1-2 times a month. If we focus on it then once a week. We make a date night and then we do it. Yep, it ain't like it was in our first years of marriage and after listening to some of my friends, it sounds like this is more the norm than unusual. In some countries the wife sets up the husband with a mistress (like Japan).
The truth is that we have to care for each other for reasons other then sex because it does go away.
Men don't need Viagra. We need new partners. Not to marry but for sex. I love my wife and I am glad to have her in my life and when we do have sex it is usually very good. Our society and moral rules (gag) confine us from having other women just as t confines our wives from having other men and still keeping marriages together. So our answer is to divorce, seek new partners and and begin the cycle again.