How hot is too hot to ride?

Maybe I'm over reacting, but I'm slightly disgusted right now. Today we hit a record temperature of 100 with moderate humidity-like 40%. Maybe that's not hot to some of you, but I live in western WI where temperatures now should be in the upper 70's/low 80's now. When we get to 90 it's horrible...so you get the point. Anyway, I went to the barn to hose off my horse's legs and put fly spray on. I feel so bad he even has to be outside in this weather....then I find out, the barn I work at had lessons today. My friend lessons/sorta works there, and she said she rode two horses and 5 others rode, along with one of the trainers and the lesson kids. There were a lot of horses ridden today. Honestly I was appalled. I felt bad just walking my horse up from the pasture. Am I over-reacting? Do you ride when it's this hot? Do you have a set temperature, where you won't ride if it's hotter than that?