I took my sim to a community lot and the option to spar showed up on everyone, even non-military sims. One of those sims is one I couldn't find the option to spar with before either, but this time it showed up. So it seems like it may be buggy. Also, it takes several times for the spar with others daily task to complete, so it's still kind of a pain.
I'm not sure sims that aren't in the military career get the option to spar with others. I switched to a household that had two sims maxed out in fitness and neither one got the option to pop up. The spar option also has the military career symbol next to it too, so I'm not sure.