Since Ebbesen's seminal work in 1998 observing extraordinary optical transmission (EOT) through nanohole arrays, much research has focused on the role of surface plasmon polaritons (SPPs) in EOT. While the energy and momentum conditions have become clear, no consensus has been reached on the role of incident light polarization. This study presents a simple model that captures Bloch-SPP excitation, including the role of polarization, in general periodic plasmonic structures. Our model predicts that under certain conditions polarization conversion should occur in EOT light transmitted through the nanohole array. We experimentally measure polarization conversion in EOT and compare the experimentally obtained results to the predictions of our model. Using numerical simulations, we tie the far field experimental results to the near field underlying physics described by our model. In using polarization conversion to provide evidence supporting our model, we also establish a novel approach to achieving polarization conversion based on SPPs instead of hole shape or other techniques in literature, and present reasons why this approach to achieving polarization conversion may be better suited for applications in biomedical sensing and optical elements.