Correct, there is no client side enforced validation. Add to Cart will default to a quantity of 1.
You have a couple of options:
1) Restrict quantity input to numeric only. Options:
b) Qty DropDown, using the built-in Restricted Quantity feature.
2) Add server side validation, so that once the form is submited the value will be validated, and any error reported back to the user on the product page.
3) Both 1 & 2.