#1 rule when recording others without their permission: DON'T!
Voyeurism is mostly fantasy, there are many laws on the books and many people who will get violent if they think they're being peeked.
There are exceptions, but they're rare. I've had friends who didn't mind if I looked, and I've shown off for others (many years and many pounds ago). This is fine for one-on-one where the person or environment suits.
But have any kind of recording equipment around, and all bets are off. Even the most show-off gym rat will get upset if there's any possibility that the moment can be put on the Web for all to see.
Almost all of the Voyeur videos we watch are posed. The bathrooms are marked 'down for cleaning' or some such, and the people let in are usually paid. Even the locker room scenes have some level of awareness.
I'm not mentioning this to stop anyone's enjoyment of the videos, but to let people know what they can't get caught doing, even in this day and age.