There may not be any laws covering this, but people tend to do a 3-point check when trying to figure out someone's gender. First they look at your face, then they look at your chest, then they look at your crotch. If people see a bulge in an unexpected place, all sorts of things could happen. It's up to you whether that's something with which you'd be comfortable. Sadly, I doubt many people would react positively.
As for (cis) women showing camel toe... most people would feel grossed out by that. People generally don't want to see the outlines of someone else's privates if it can be helped, so camel toes tend to be frowned on too! Most women go out of their way to avoid having a camel toe; if you're curious, sanitary towels are one of the most common ways of preventing it.
If women are displaying an (unavoidable) camel-toe during a yoga class, that's generally because they're in an environment that's mainly geared towards women, and everyone is facing the same direction. So they certainly don't expect anyone to stare at their privates in what they consider to be a safe space... hence the lowering of their guard.
Incidentally, there are clothes you could wear that would allow you to go untucked without any visible bulge (e.g. certain types of skirts and shirts).
