Here in the Houston are,a I routinely ride in temps. of over 100 degrees. Often the humidity here is 90% or more, so we're hit double both heat and humidity. Most of my rides on those days are 25 miles or less. I usually wait until the late afternoon and ride into dusk. As long as I can spend much of my ride out of direct sunlite i'm happy. But I have ridden at mid-day with those same temps too.
With that being said I would MUCH rather ride on a 100+ day then on a 50 degree or less day and my hat is off to all those I see on a cold day riding.