In general, the softer the seat, the worse the problem, particularly chafing. I ride a Brooks Pro saddle (for 37 years), which is simply a piece of tensioned leather, and have ridden it for 6+ hours in the past (far distant past) with just gym/running shorts and no sores. Not sure I would try that now, but I can ride it over an hour without using bike shorts and no real problems. As mentioned, biking underwear might be an option if you don't want to join the lycra crowd. It also depends on how upright you are sitting. Distributing your weight between your butt and your arms/hands will alleviate some of the butt pain (and possibly introduce wrist pain). Are these normal spin bikes (Schwinn whatever, or the like)?