We generally go to Disney in mid-June and have gone these past two years. It can be extremely hot. This year there was a heat wave and it was unbearable, felt like over 100 everyday. Basically from the instant you stepped outside you started sweating. The June before was not as bad, but some days were very stuffy and also very hot. Temps in June are well into the 90's or will at least feel that way.
That being said, I think a short ceremony outside would be okay, but I would recommend you do an earlier ceremony time in a location that has some shade.