I don't see it Dodgy - If it takes X amount for a decent shower and my kids use 10X then ten times the water is pumped here by the waterboard, ten times the gas heating, ten times electrically pumped upstairs, bathroom lights and fans are on longer than needed. Then there's the waste water to be purified too. That is a lot of needlessly wasted energy...
You are quite correct that if you use 10x of anything, then the absolute costs are going to be higher than 1x of something. But your comment that this is 'needlessly wasted energy' takes us into a whole new ballgame of discussion, more appropriate for a book than a thread here! Remember, my initial point was not that using energy was not expensive, but that it was not 'an environmental cost'.
If you consider the costs of 'standardly provided water' vs the same amount of water provided in a 'green fashion', it is relatively easy to show that with the huge costs associated with things like local building rainwater capture, storage and dual-pipe 'grey water' use, reservoirs and mains piping are far cheaper and (if you want to believe that CO2 and energy use is bad) much more 'environmentally friendly'. The maintenance costs of filter changing and chemical anti-algae treatment for dual-pipe systems alone are horrendous. In fact, reservoir water storage, purification and sewage treatment, once set up (as the Victorians did for us) is a very cheap process.
But you have widened the field to consider general energy costs. Again, you are correct to imply that at the moment these are very high, and if you want to save money it is appropriate to cut its use as far as possible. My concern is that energy costs are high, not because that is the cost of energy production, but because environmental activists want people generally to cut energy for idealistic reasons - religious, if you will - and have persuaded politicians to increase costs considerably via taxes in order to achieve this. During the 1950s it was believed that nuclear generated electricity would become 'too cheap to meter', and that energy would be, for household purposes, effectively free. There is no technological reason why this should not be the case - the base costs of nuclear generation are very low - but determined activism coupled with capitalist love of profit has meant that society has kept energy costs artificially high. Current 'green' taxes are pushing this to the brink.
So I agree that using 'a lot' of energy is currently expensive - but maintain that it is expensive for political reasons, not fundamental technical ones, and not environmental ones.
Is having an hour-long hot bath 'wasting energy'? It is certainly expensive for the reasons I have given above - and that may be all that is needed to close the argument if it is your wallet paying for it. But I am a follower of Julian Simon's Cornucopia theory, and note that personal energy use has relentlessly increased throughout the ages. In 1900 a person would be controlling about 750 watts if he was traveling somewhere - by 1950 that figure would be about 20Kw, and today we think nothing of using 100Kw. Simon's theory stresses that each generation must and will use more and more resources, and that they will never run out. Google it for details, or have a look here:
http://www.webcitation.org/5Xu64dbNz A bit of a read, but well worth it...
Your sons are just following Simon's laws. And the energy isn't 'wasted' - for their generation it's the norm. That's what Simon means when he says that:
"..the material conditions of life will continue to get better for most people, in most countries, most of the time, indefinitely..."
What would your grandfather have said if he could see you running central heating on a cold night? I'm guessing that a small coal fire would have been good enough for him? His 'waste of resources' is today's norm, and, so long as environmental activists don't get their way, tomorrow's humans will laugh at our meagre lifestyles...