It’s not legal. What it comes down to is that some states wrote laws saying any sexual encounter where one person is in duty in a position of power, that encounter is automatically deemed non-consensual. In other states, it would follow the exact same laws as any other sexual assault. In these other states, if a rape were reported, during the ensuing investigation/court case (if there actually was one), the prosecutor would basically just say the same thing, that there was no consent because of the power dynamic. I don’t know whether or not those laws make a difference in the long run. It’s probably a good thing to have on the books, but not as sensational as people make it out to be.
To use the first weird analogy that came to mind, it’s like if one state had a law saying that you couldn’t poke whales with sticks, while another state just said you couldn’t mess with whales. Either state would prosecute you for poking a whale with a stick, just the one would have to say that poking is a form of messing with the whale.
There’s arguments for and against laws like that, cause on the one hand, you want laws to be as simple as possible to cover whatever use case, but on the other hand, you don’t want to give criminals/businesses/etc. to have any wiggle room to do things against the public interest because there isn’t a specific law against it