The problem is, of course, that those same people in the gym—an environment which is naturally quite conducive to spreading infections—later go to their workplace, or their doctor, or their public transport, or a bar, and back to the gym. By doing that, they can carry an infection around to others, even if they aren’t themselves symptomatic.

This contributes to the general public health problem, increasing the infection rate among the population and meaning more people get sick or die. This applies whether they were in the gym, or just had the misfortune of interacting with someone who had been.

I don’t know who initially said it, but… I don’t know how to explain to you that you should care about other people.

Nobody particularly cares about people doing high-contagion activities like going to a gym, for the people in the gym themselves, the ethical problem is that they're very few degrees of separation away from people who can't afford to be exposed. Even if you drive by and mind your own business, that gym almost certainly has a blast radius of unnecessary infections and deaths.

higher chance of infection. you don't live in a vacuum.

nah, shut you down with the health board for putting everyone's lives at risk. there's no live and let live here. if folks just sit back and do nothing, people will die.