If you’re going to do some form of sport or activity where you would be sweating so much that you would want to be without tzitzis, you are allowed not to wear them.
However, every serious frum rov will simply tell you if you ask him, that in that case, you simply shouldn’t do that sport/activity. Obvious solution.
As for me, there have been times in E”Y on extremely hot days when I went without tzitzis because I literally got sick from the heat quite a few times. I can’t stand heat very well.
This summer I’ll be in Switzerland, walking in the mountains. Depending on the temperature, I’m initially going to keep my tzitzis on as usual. Should it become extremely hot, there is always the option to put them in my backpack.