Of course bras get sweaty, unless you're completely sedentary.
People are physiologically variable and live in different environments. I can cycle 10 miles on a push bike and not sweat, so long as it's a cool day and I keep my speed down to a comfortable commuting pace. I do sweat more when less fit but in general find moderate activity in the usual UK climate doesn't make me sweat. I appreciate that others may sweat more, or less, than I do.
I wonder if all the ‘what about the environment!’ wailers can honestly say they’ve never put their wain in a disposable nappy (for up to 4 years each according to MN toilet training trends) or used a wipe or a nappy sack.
Well a. it's not 'wailing' to express legitimate concerns about what we're doing to the environment. No amount of underhand rhetoric will help us with the crisis we're creating, although it may help you wilfully ignore it. And b. I went one step further and just didn't have children. Sure, on a site called Mumsnet, that won't be the popular option. But actually yes, there are many other environmental choices I make, including commuting by push bike. I'm also careful about food miles and food wastage, ensure I reduce, re-use and recycle, in that order, have cut out single-use plastics as far as I can and make use of solar and wind power whenever possible.