Bananas do have seeds. If you look closely in the centre of a ripe banana, all the way down the very middle of the banana you will see little black specks. These are the seeds. You can order banana seeds from places on the web such as RarePalmSeeds.com.
One can only speculate why anything is the shape it is, including branches, leaves, and why we have four limbs instead of six, etc. Nobody knows for sure. I'm pretty sure it's not because of gravity or sunlight. It probably has something to do with the closest way to pack fruit into a given space so that all the fruit can share a point of attachment. Also, not all bananas are curved, some are pretty straight. There are lots of different kinds and shapes of bananas. The key point is: we don't know why they are the shape they are.