Probably depends on the area. If Texans stopped riding when it got hot and/or humid, we'd only ride for a few intermittent weeks between September and April.
It's been in the 90s here all June and the MUP is at least as crowded as usual.
But I rarely ride the MUP so it doesn't affect me. I prefer the open rural routes and nearby access roads with little or no traffic. I still see the same handful of joggers, walkers and other cyclists as usual.