No, it isn't. And OP, this forum isn't representative on this issue (of Americans, in general). Not only is spanking legal but it is widely socially acceptable.
I have spanked my kids. I've got five, four biological and one stepdaughter that I've raised since she was a toddler. However, I haven't spanked one of them in almost 3 years. My wife spanks too, and I think it has been 2 years for her. Some kids, spanking modifies behavior. Some kids, it doesn't. Growing up, I got the belt, and my younger brothers got worse. My father was a tyrant and our family was terrified of his rages. My kids don't walk around in terror of me, we have a good trusting relationship, and it's a far different situation. Physical discipline can help, particularly younger children who can't be reasoned with, or a hysterical child past listening.
Within the boundaries of the law, physical discipline is something that each parent has the right to decide for themselves and their children.