  We present the main sequence for all galaxies and star-forming galaxies for a
sample of 28,469 massive ($M_\star \ge 10^{11}$M$_\odot$) galaxies at cosmic
noon ($1.5 < z < 3.0$), uniformly selected from a 17.5 deg$^2$ area (0.33
Gpc$^3$ comoving volume at these redshifts). Our large sample allows for a
novel approach to investigating the galaxy main sequence that has not been
accessible to previous studies. We measure the main sequence in small mass bins
in the SFR-M$_{\star}$ plane without assuming a functional form for the main
sequence. With a large sample of galaxies in each mass bin, we isolate
star-forming galaxies by locating the transition between the star-forming and
green valley populations in the SFR-M$_{\star}$ plane. This approach eliminates
the need for arbitrarily defined fixed cutoffs when isolating the star-forming
galaxy population, which often biases measurements of the scatter around the
star-forming galaxy main sequence. We find that the main sequence for all
galaxies becomes increasingly flat towards present day at the high-mass end,
while the star-forming galaxy main sequence does not. We attribute this
difference to the increasing fraction of the collective green valley and
quiescent galaxy population from $z=3.0$ to $z=1.5$. Additionally, we measure
the total scatter around the star-forming galaxy main sequence and find that it
is $\sim0.5-1.0$ dex with little evolution as a function of mass or redshift.
We discuss the implications that these results have for pinpointing the
physical processes driving massive galaxy evolution.
