*Some exceptions apply.
You should never touch someone without their permission — this is a rule of life and is true 99.9% of the time. The only circumstance where this rule gets challenged is in a lights-out space made for anonymous play — spaces which exist in some leather bars, gay sex clubs, gay bathhouses, and so on.
In these spaces, you waive a degree of consent by entering them, and consensually submit yourself to being touched by people you cannot see. This is why we enjoy these spaces. They can be uncomfortable for people who don’t understand what they are or aren’t prepared for them.
In all other cases, never touch someone without their permission — even and especially in a leather bar, when you may be tempted to assume a “free pass” for touching because of the heightened sexuality of the space. Sure, on some dance floors, with some substances, people get touchy-feely. You know how to do this, you’ve navigated this before — that dance floor feeling when you’re crowded together and everyone’s hand is on everyone’s crotch. While it’s difficult to parse through where consent falls in situations like this, it’s wise and respectful to maintain the importance of asking first in all situations.
If someone is tied and gagged and being led around via collar and leash, ask the person holding their collar. If you’re in a sling room and a guy is in the sling waiting to get fucked, he’s giving nonverbal consent to at least come inspect him. It’s still polite to ask before touching him, but by being there he’s giving a degree of consent to be viewed. (If he’s blindfolded, as I often am, he’s forfeiting a hefty amount of consent — he’s willfully handing over the reins. Consent gets nonverbal in situations like this.)
At first glance, kinky sex spaces make consent murky. Upon second inspection, you’ll see where consent is given. In some cases, it’s given by simply setting being in a particular situation, such as with lights-out backrooms. In all other circumstances, never touch someone without their permission. This is the fine line between consensual sex play and assault. The kinky community does not condone assault and never will.