No jeans in first class? Bull. I fly AA quite a bit. When I fly I dress to be comfortable, not to impress other passengers. I have flown first class on international flights many times wearing jeans.
Usually when I fly to the DR I wear plastic exercise pants that are really comfortable. I have worn these on a ton of DR flights. I usually fly coach but even in first class I never have a problem. In fact because I just missed going platinum last year, I get upgraded to first class once in a while for free. A few times I have been sitting comfortable in coach waiting to take off and a flight attendant will come and get me. They take me to first class if there is an open seat, sweat pants and all.
I also do not notice even a slight bit of difference in service. I used to fly so much that the novelty has worn off years ago. I am usually asleep before the plane takes off.
As far as shorts in the DR. I wear them from time to time in the daytime. I will walk down El Conde wearing shorts, T shirt and sandals. I don't get any funny looks and could care less if I did.
Of course I don't try and enter museums, nice restaurants etc. I don't see the need for a collared shirt and long pants to go to KFC, an internet cafe or to have a beer at Pacos.
Night time is a different story. I still wear jeans quite a bit, but if we are going out on the town, I am not opposed to puting on a blazer or suit. It all depends on where we are going and how dressed up the novia is getting. If she is wearing a long gown, I will wear a suit. If she is wearing a short sexy dress, maybe jeans with a silk T and a blazer. Never a tie;-)