Is it unhealthy to play sports when it's 105 degrees outside?

Here in south Texas, I can't get anybody to play tennis or basketball during the heat of the day. Yes, it does hit 100 almost every day and even 105+ at times. When I go out, I take a gallon of water and three bananas not to mention that I have a nutritious meal prior to exercise. My friends say that it is unhealthy to be out there - I disagree. Who's right?