Depends. I commute on my bike, but if it's going to be humid and I have a trip or a meeting, I won't take the bike if I'll be sweaty/stinky by the time I get to work. Here in Northern Ohio where the humidity has been way "up there" lately, the upper 80s seem to be that limit.
If I don't need to interact closely with others, then anything above 100 becomes non-fun, above 110 is a no-go here. If I'm traveling, there's no real limit. I'll do my best to stay hydrated and reasonably comfortably. When I can't do that anymore, I change the game plan.