Through the study of a carefully selected sample of isolated spiral galaxies, we have established that two important global physical quantities for tracing star-forming activities, LFIR and MH2, have a nonlinear dependence on another commonly cited global quantity, LB. Furthermore, we show that simple power-law relations can effectively describe these nonlinear relations for spiral galaxies spanning 4 orders of magnitude in FIR and MH2, and nearly 3 orders of magnitude in LB. While the existence of a nonlinear dependence of MH2 (assuming a constant CO-to-H2 conversion) and LFIR on optical luminosity (LB) has been previously noted in the literature, an improper normalization consisting of simple scaling by LB has been commonly used in many previous studies to claim enhanced molecular gas content and induced activity among tidally interacting and other types of galaxies. We remove these nonlinear effects using the template relations derived from an isolated galaxy sample and conclude that strongly interacting galaxies do not have enhanced molecular gas content, contrary to previous claims. With these nonlinear relations among LB, LFIR, and MH2 properly taken into account, we confirm again that FIR emission and star formation efficiency (LFIR/MH2) are indeed enhanced by tidal interactions. Virgo galaxies show the same levels of MH2 and LFIR as isolated galaxies. We do not find any evidence for enhanced star-forming activity among barred galaxies.