Obviously this is a porch swing, it is for two adults and the front and back of the seat are rounded to fit your body right.
This picnic table is made of 2x6 lumber and there are no visible fasteners, no screws in the top or seat. We have often been asked if we would build it with treated lumber, the answer is no. We just don't like working with it. Have you ever actually looked at the warnings of that stuff? We will put a coat of Thompson's water seal on it. Then it can either be painted over or left to see the grain.
Our Adirondack chairs are also rounded on the back and front to fit your body right. We have had many people with back problems tell us they are very comfortable for them. We have two on our own deck and I enjoy every minute I get to sit out there.