  The slope of the star formation rate/stellar mass relation (the SFR "Main
Sequence"; ${\rm SFR}-M_*$) is not quite unity: specific star formation rates
$({\rm SFR}/M_*)$ are weakly-but-significantly anti-correlated with $M_*$. 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 $M_*$.
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 $({\rm sSFR_{\rm disk}\equiv SFR}/M_{*,{\rm disk}})$ reduces the
$M_*$-dependence of SF efficiency by $\sim0.25$ dex per dex, erasing it
entirely in some subsamples. Quantitatively, we find $\log {\rm
sSFR_{disk}}-\log M_*$ to have a slope $\beta_{\rm disk}\in[-0.20,0.00]\pm0.02$
(depending on SFR estimator and Main Sequence definition) for star-forming
galaxies with $M_*\geq10^{10}M_{\odot}$ and bulge mass-fractions
$B/T\lesssim0.6$, generally consistent with a pure-disk control sample
($\beta_{\rm control}=-0.05\pm0.04$). That $\langle{\rm SFR}/M_{*,{\rm
disk}}\rangle$ is (largely) independent of host mass for star-forming disks has
strong implications for aspects of galaxy evolution inferred from any ${\rm
SFR}-M_*$ relation, including: manifestations of "mass quenching" (bulge
growth), factors shaping the star-forming stellar mass function (uniform $d\log
M_*/dt$ for low-mass, disk-dominated galaxies), and diversity in star formation
histories (dispersion in ${\rm SFR}(M_*,t)$). Our results emphasize the need to
treat galaxies as composite systems -- not integrated masses -- in
observational and theoretical work.
