I always see these two associated, but I haven't yet had the chance to learn. How are they related, and why are they always paired together in forums?

 

They are the same platform.

 

Similar to the F-Body cars? I remember the Camaro and the Firebird being built on the same platform.

 

More or less.

They share a lot with the same year Civic as well but the standard Civic wasn't K powered and these were the first of the K stuff.

 

As others have said, they share a "Platform" which is engineer/manufacturing terminology for "the same, mostly, except for the parts that are not" or "we started from some common basis and adapted as needed". Those answers are not very satisfying though, so here are a few specifics I have become aware of over time (generally I am talking about the 02-03 EP3 and 02-04 Base RSX but there are similarities for the other years too):

• Base RSX and EP3 have the same K20A3 motor, except the Base RSX has a less-nice intake manifold compared to the PRB on the EP3
• Base RSX and EP3 have the same front lower control arms and the knuckles and struts and axles are close enough you can swap them around at will (but RSX is 5 lug while early EP3 is 4 lug, so hubs are different)
• RSX rear trailing arms are almost the same as EP3 except the shock mount is slightly wider for some reason (you can shave or shim the damper bushings to interchange the rear shocks too)
• The B-Pillar brace under the carpet can be swapped between the RSX and EP3

So in summary, with the front and rear suspension being close enough to move most of the parts back and forth, and the floor being the same dimensions at the B-pillar, and nearly the same motor you can see how similar the cars are.