ok well look at the songs that are indisputably 90's r&b ballads subject matter...

somethings going on
breaking my heart
come back to me
in my bed
unbreak my heart
don't let go
etc.

none of those are as a direct as something like "sex me", "nice and slow" or "nobody".
despite similarities like the key, scale, lead instruments, arrangement and tempo.
the subject matter of the songs i mentioned often tackles being rejected and relationship issues.
i think that is a clear characteristic difference between 90's slow jam and 90's ballad.
there are exceptions like early jodeci
but they are just barely toeing the line from being categorized as slow jams.