The slope of the star formation rate/stellar mass relation (the SFR "Main Sequence"; SFR-Mlow *) is not quite unity: specific star formation rates (SFR/Mlow *) are weakly but significantly anti-correlated with Mlow *. Here we demonstrate that this trend may simply reflect the well-known increase in bulge mass-fractions-portions of a galaxy not forming stars-with Mlow *. Using a large set of bulge/disk decompositions and SFR estimates derived from the Sloan Digital Sky Survey, we show that re-normalizing SFR by disk stellar mass (sSFR sub(disk) = SFR/M sub(low *,disk)) reduces the Mlow * dependence of SF efficiency by ~0.25 dex per dex, erasing it entirely in some subsamples. Quantitatively, we find log sSFR sub(disk)-log Mlow * to have a slope beta sub(disk) setmembership -0.20, 0.00 + or - 0.02 (depending on the SFR estimator and Main Sequence definition) for star-forming galaxies with Mlow * > or =, slanted 10 super(10) M sub(middot in circle) and bulge mass-fractions B/T <, ~ 0.6, generally consistent with a pure-disk control sample ( beta sub(control) = -0.05 + or - 0.04). That left angle bracketSFR/M sub(low *,disk))right angle bracket is (largely) independent of host mass for star-forming disks has strong implications for aspects of galaxy evolution inferred from any SFR-Mlow * relation, including manifestations of "mass quenching" (bulge growth), factors shaping the star-forming stellar mass function (uniform d log Mlow */dt for low-mass, disk-dominated galaxies), and diversity in star formation histories (dispersion in SFR(Mlow *, t). Our results emphasize the need to treat galaxies as composite systems-not integrated masses-in observational and theoretical work.