I have riden in 90f weather with and without a jacket, and you seriously stay cooler with a jacket on. People dont get that the body natural cooling mechanism is your sweat. You may sweat in the jacket, but going at speed, it will evaporate and cool you. Without a jacket, the sweat just evaporate instantly and it doesnt even have a chance to cool you off. You may feel like you are getting cooler without a jacket, but thats just the air rushing over the skin. Your internal temperature will be higher without a jacket because of no sweat cooling and you can get a heat stroke. There is a reason why people in the desert and hot places in africa wear long clothings.
I do sometime not wear a jacket when I ride, but I try to limit it to when I am just riding around in the city. I will always wear my jacket riding to work because I dont like rocks hitting me in the chest or arms, and its all highway cruising at 80mph.
I always wear my gloves and long pants. The only time I will not wear a helmet is just cruising through the city under 30mph, or a very short ride of 10mins or less.
The only problem with wearing a jacket is in high humidity with high temperature.....that is just straight torture and I try not to ride, but if I do, I will zip down my jacket halfway to get some more air in.