I know how sex works and so does the majority of the world. So why do celebs need intimacy coordinators? I mean can't the guy wear a cup and the female wear a skin colored thong?

Independent third party to ensure boundaries are set and clearly communicated between the actors, and to observe the act to ensure process is followed and to stop things if they are not.

If you are referring to actors and intimacy coordinators, I think I have you covered.

Sex isn’t just a physical act, it is an emotional one. Acting is taking a script and adding believable emotion to physical actions while staying true to the scene and what the director wants. Sexual behavior can cause emotions to rise and bring out spontaneous or unpredictable actions and feelings, which can ruin scenes and traumatize actors.

Intimacy coordinators are present in the acting industry to help actors navigate the unexpected feelings that may arise and keep actors safe. The last thing you want to happen while filming a rape scene when actors are channeling really intense emotions is for real violence to happen. That is why intimacy coordinators are very very important beyond merely dressing actors to prevent sexual contact in sex scenes.

Sex isn't the only kind of intimacy.

Ok, so imagine you were expected to strip naked, cuddle up to someone else who was also stripped naked. Imagine you are expected to pretend to have very real and intimate feelings with one another and in the process do very intimate things with them like kiss, touch their body, and simulate penetrative sex all while being in a room full of people watching you do this and with cameras recording the entire thing with the intent to show it to the world. That in itself would make most people at least a bit uncomfortable.

Now, maybe you also feel uncomfortable because you have a spouse or significant other. Maybe you know this other person well and they are platonic friends. Maybe they are barely an acquaintance, just a work colleague. Maybe you have romantic feelings for this other person. Maybe you hate them. Maybe the other person has made advances on you. Maybe you have a history of being assaulted or of being used sexually. Maybe you are self conscious about your body. For any number of very justifiable reasons, this situation can be anywhere from slightly to very uncomfortable for either or both actors, even scary in some situations.

The intimacy coordinator is not a sex coach. They are there to make sure the situation is safe, consensual, and as respectful and private as possible for both actors, and to see that their needs are met for this very stressful circumstance to minimize discomfort. They also make sure that nobody on set oversteps boundaries, that the actors' rights are upheld, act as the single point of contact between the actors and production to minimize uncomfortable conversations and miscommunication. They will be involved in choreography of the scene, but not as a coach so to speak, but rather in mentally preparing and making exactly clear the understanding for all parties what is going to happen, giving everyone the time to process and veto plans if necessary.

They’ve had the physical stuff covered for a long time, but imagine the socially and emotionally charged atmosphere that must come with having to film something personal and intimate with a coworker. Both actors might have spouses at home, maybe they’re super close friends off camera, maybe they’re playing a different sexual orientation than their own. Who knows. People are messy. Everyone thinks about sex a little differently, so when you add all these things into a work environment, having a third party there whose job it is to prepare the actors and crew for exactly what’s going to happen when the camera is rolling, keeps things professional and makes sure each actor’s wishes are respected. Seems like a win for everybody.

If it makes everyone involved more comfortable, they’ll probably do a better job at making whatever tv show or movie they’re on, and it will be more entertaining for us. Plus, we as viewers can be relatively confident that none of the actors did anything they didn’t want to do.

It's important for the same reason a stunt coordinator is important. It's not about covering genitals. By having a controlled environment of rehearsed choreography in a professional setting with trained supervision, it ensures the performers can act out their scenes with full consent and safety. This allows the director, director of photography, and necessary crew can do their jobs efficiently. It also protects the entire production from issues of liability, which is important for not just legal reasons but also insurance costs.

Sex can be more than just the physical act. There are often feelings or misconceptions (good and bad) that can come up during the act, even if it is only being acted, and especially if it is going to be filmed in front of a crew of any size. An intimacy coordinator helps all parties involved (actors, directors, sound, lighting) feel comfortable and not work outside their own safe boundaries.

There has also historically been people who take advantage of others based on their perceived power in the film industry. An intimacy coordinator is there to stand up for the safety and rights of people on set when they are at their most vulnerable.