  Spectral synthesis is a powerful tool for interpreting the physical
properties of galaxies by decomposing their spectral energy distributions into
the main luminosity contributors (e.g. stellar populations or ionised gas).
However, the impact nebular emission has on the inferred properties of
star-forming (SF) galaxies has been largely overlooked over the years. The
objective of this work is to estimate the relations between stellar properties
of SF galaxies from SDSS DR7 by simultaneously fitting the stellar and nebular
continua with FADO and comparing them to the results derived using STARLIGHT, a
representative of purely stellar population synthesis codes. Differences
between codes regarding average mass, mean age and mean metallicity values can
go as high as $\sim$0.06 dex for the overall population of galaxies and
$\sim$0.12 dex for SF galaxies (galaxies with EW(H$\alpha$)>3 \AA), with the
most prominent difference between both codes in the light-weighted mean stellar
age. A closer look into the average light- and mass-weighted star formation
histories of intensively SF galaxies (EW(H$\alpha$)>75 \AA) suggests that
STARLIGHT is underestimating the average light-weighted age of intensively SF
galaxies by up to $\sim$0.17 dex and overestimating the light-weighted
metallicity by up to $\sim$0.13 dex compared to FADO (or vice versa). The
comparison between the average stellar properties of passive, SF and
intensively SF galaxy samples also reveals that differences between codes
increase with increasing EW(H$\alpha$) and decreasing total stellar mass. This
work finds indirect evidence that a purely stellar population synthesis
approach negatively impacts the inferred stellar properties of galaxies with
relatively high star formation rates. In turn, this can bias interpretations of
fundamental relations such as the mass-age or mass-metallicity.
