Where are you from? I'm from central Texas, and it's hotter here than at Disney in the summer, so we have no problem going in July. The key to enjoying Disney in summer is to take breaks in the afternoon at your resort (get one with a good pool), wear water-friendly shoes for those late afternoon showers (it cools off a lot after those), wear loose-fitting clothes (avoid denim, which tightens when wet), sunglasses, and a wide-brimmed hat or a UV umbrella/parasol. Drink plenty of water.
Preparing for summer in the South is just like preparing for winter in the North. You wouldn't visit Aspen over Christmas wearing open-toed sandals and no coat, or expect to sit outside in the snow for eight hours straight. Likewise, for an Orlando summer, you should bring summer-appropriate clothing, and also plan your days to include break times indoors or in pools during the hottest time of the day, rather than forcing yourself to push through the heat all day.
If you're prepared, summer at Disney is great. The warm nights are lovely, and the swimming is awesome. It will get hot mid-day, but it isn't Kuwait or the Sahara Desert. You'll enjoy yourself.