Well, yeah, lots of the modern day eco-warriors are childless or single, young people - but I bet you they drive and fly around like there's no tomorrow! They will not compromise their lifestyles - they just want you to compromise yours!
We do have a wood burner just because we like the look. However, we don't use it much and I don't think most people who have one use it on a daily basis either, unless it's their main heat source. Reason being, it's difficult to keep it clean and wood is quite expensive. It's just a nice focal point in the room - we've had it on perhaps 4-5 times this winter so far.
When we look at environmental impact, we need to look at the whole picture. What else is that person doing or not doing to impact the environment? I think some people are way more polluting than others, and I wouldn't be surprised if there were some eco-warriors among them! There should be some kind of a scale to add up your overall carbon footprint, with varying degree of points for the activities people engage in.
Out of curiosity, I've done a carbon footprint test. My score is 9.4 tonnes, which apparently is just under the UK average. That's with eating very little meat, walking to work and taking no flights in the last 12 months, and recycling everything I can. We live in a larger house, so I think that was a negative factor. It's a bit misleading though because they ask you how many adults are in the household - it should absolutely be the total number of people, including children, in my opinion.
The world average is 6.3 tonnes. Here's a link if you want to try it: footprint.wwf.org.uk/#/