Um, sorry to butt in, but there's a lot of **** being talked here.
First, the material used to build a frame is only one of many factors to consider. There is no inherent weakness that will prohibit you from using carbon fibre. **** frames can be built from any material. I can think of lots of steel, aluminum or titanium frames that would be unsuitable for you for reasons that have nothing to do with the material (i.e. poor design, poorly-drawn tubing, bad welds, etc.).
Second, there are reasons for choosing carbon other than weight savings. You are all correct in stating that to a large extent it is the weight of your bodies - not your bikes - that is the limiting factor in your riding. But anyone who says that carbon does not offer a different ride characteristic than other materials - not necessarily better, but certainly different - has never ridden a carbon fibre frame.
I currently own steel bikes and a carbon fibre bike, I have had aluminum (but would never buy again - worst rides I have ever experienced), and I have had titanium. In three years I have ridden over 12000km on some pretty rough roads (our cobblestones are world-famous), most of which was on my carbon fibre bike, and it is still in fine fettle. It was even x-rayed three weeks ago to ensure that there were no problems.
I bought carbon because in my country, on my rides, carbon offers better absorption than aluminum, better stiffness than steel or aluminum. I think you need to think long and hard about what you want from your bike, and if carbon is the best material for you, then don't hesitate. Just make sure you buy quality, irrespective of the material.
Good luck!
HW