I was glad to see Fred's post, and Marco had a good point about the older buildings.
I guess I'm in a slim minority here, but I don't much care for air conditioning. I like to open the window, smell the local air, and get acclimated to local conditions. Even in the Southwest US. (My sister lives in Phoenix. When we visit her we turn off the a/c in our room and open the windows.)
And as a few people have indicated, most of Europe, especially north of the Alps, doesn't get terribly hot. Yes, every country has the occasional aberrant heat wave, but on the whole, summer temps are moderate.
Edit to add: I do know that there are medical conditions, such as MS, that make people unable to tolerate heat. I'm just commenting on my personal preferences. And yes, I have already realized that if I ever take a tour without my DH, I'll have to pay the single supplement!