At my former gym, I'd say there were as many people wearing full kits as those wearing a combo of bike shorts and other tops.
It depended on what class you went to. Some classes had more outdoor riders in them, especially the ones that were added in the winter, for "training," and were geared for cyclists. If I happened to go at 9:30 AM, well, it was the housewife crowd. But, most of them wore bike shorts.
I couldn't sit on those seats in regular shorts.
2015 Trek Silque SSL
Specialized Oura
2011 Guru Praemio
Specialized Oura
2017 Specialized Ariel Sport