I think the error in logic is there is a jump from less consumption to we need to ban plastic bags. It seems too sudden and a little extreme. Maybe a slower transition and less abrasive conclusion would help. How about this:
The whole city is littered with trash, and we need to do something about it. It's so bad that garbage is floating in the ocean and being washed up on our shores. If we can control our consumption and limit how much trash we generate, then we'll be able to combat this menace. One of the main pollutants to our city is plastic bags. Plastic bags are very harmful to ocean animals and the overall environment. I think that plastic bags should be banned because it is the best way that we can clean up our city streets. We should ban plastic bags so the trash doesn't become so bad that we are living in filth and squalor.