They do want the same quantity though, you simply need to understand this. It doesn't matter whether I am married yet or not, but women love sex. They are probably losing their sex drive because over the course of living together for that long, the mystery and sexiness of their partner lessens. It doesn't need to be this way.
And while you might know about vibrators and such, I am more than willing to bet the less than a quarter of the guys here have ventured out and purchased something of that variety to spice up the sex life a little. I can't imagine how boring sex with the same woman would be after 20 years, regardless of how good she is in bed. We would eventually need to try something to spice it up, and if many of the couples here are stuck on plain vanilla sex, try something new, just go for it.
And aneconomist, don't talk down to me here either (bub?). There have been some sob stories here and I am sincerely doubt that many of the guys have done more than bitch, moan, and beg. Simply picking up some of your girl's chores won't do it. Make her want you. Think about before you were married, would she want to f*** simply because you acted like you cared and did her laundry? No.
I would have agreed with everything you have said before i was married. After being married for many years it can become very tricky to keep that fire. I would have never guessed that to be the case until i lived the married life for several years. Killer sex is tough to come by no matter how hard most couples try after 10 or 20 years together.