Well, using paper costs more water than using water itself. Also, the ecological problems of using paper is far worse than using water alone[0][1].
[0] https://en.wikipedia.org/wiki/Environmental_impact_of_paper
[1] https://en.wikipedia.org/wiki/Toilet_paper#Environmental_con...
Intuitively, how much water do you think the toilet paper of one bathroom visit would heat, if usef as fuel? (Answer: much less than what you would use to replace TP)
I got curious and did a napkin calculation.
A sheet of TP weights 1.5 grams and you might generously use 10 sheets per visit. Assuming fuel properties of wood, that's 250 kJ of heat energy when burned. That's enough to heat only 1.5 kg of 10C mains water to body temperature.
Also just thinking about how much energy is in the TP doesn’t account for shipping it around at all.
I have no idea what the energy balance is, i just think this thread had vastly oversimplified.
Even the water volume may or may not be an issue. If you live somewhere that water is precious it’s a much different concern than if your location uses surface water that just ends up back in the same river downstream after a trip through your plumbing, for example.