Does a polygon only have to have one straight line?

Although a polygon is usually thought of as a two-dimensional figure with at least three sides, there is nothing in the definition of a polygon that prohibits its use for the classification of a one-sided figure. A one-sided figure is known as a henagon or a monogon. A polygon, by definition, has as many vertices as it has sides, so a single-sided polygon would have to, in order to meet that criterion, curve in on itself so that the two ends of the side meet to form a vertex. In order for the polygon to not be curved, it would have to be made up of two straight lines (this figure is known as a digon). So no, a polygon cannot be a single straight line.