Do I have to tell my therapist the exact thoughts?
The fear of saying the thought out loud keeps many people with intrusive-thought OCD from ever starting. Here is what actually happens in the room, and at whose pace.

You have thought about getting help. You have looked at pages like this one, maybe more than once. And every time, the same thing stops you: to get help, you would have to say it. The actual thought. Out loud, to another person, who would then know.
I want to talk about that directly, because the fear of saying it is one of the main reasons people with intrusive-thought OCD go years without treatment, and because what happens in the room is not what the fear predicts.
What the fear predicts
The fear says that if you describe the thought, the therapist will be alarmed. That they will see it as a confession rather than a symptom. That they will judge you, or report you, or quietly conclude that you are the thing the thought describes. It says that some thoughts are too bad to be OCD, and yours is one of them.
I have heard this from people whose thoughts were about harming their children, about sexual themes they found abhorrent, about blasphemy, about violence, about doubts over who they were. Every one of them believed their thought was the exception. None of them was.
What happens
Intrusive thoughts in OCD are unwanted mental events that a person is fighting, rather than threats or plans, and describing them is a normal part of treatment. A therapist who knows OCD understands the difference between a thought someone is tormented by and an intention someone holds, and the difference is not subtle. The torment is the tell. Nobody who wanted to do the thing would be sitting in front of me desperate to get rid of the thought of it.
So when you describe it, what you get is recognition. I have worked with these themes many times, and I will very likely have heard something close to yours before. It will be met as what it is: a recognised, treatable presentation of OCD.
At your pace
You do not have to say it all in the first session. Many people start with the theme in general terms, "it's harm thoughts" or "it's about my relationship" or "it's religious", and share more as trust builds. That is fine, and it is enough to start with. The detail helps, because the treatment works on your specific thoughts rather than on OCD in general, but it comes when you are ready for it, not before.
What I would gently say is that the detail usually arrives sooner than people expect. Saying the general theme and finding that the room does not collapse does something. The next session, more comes. By the third, most people have said the thing they were sure they could never say, and describe it afterwards as a relief so large it surprised them.
Why saying it matters for the treatment
The treatment for intrusive-thought OCD is CBT with exposure and response prevention, and one of its central ideas is that a thought held in secret, fought in secret, gains power from the secrecy. Every thought you have never said out loud has only ever existed in your own head, where the alarm is loudest and there is nobody to notice that the alarm is disproportionate.
Saying it is, in a sense, the first exposure. The thought exists outside your head for a moment, and you discover that the thing you feared, being seen as the thought, does not happen. That discovery is part of what loosens the grip.
What I will not do with it
I will not reassure you about the content. I will not spend the session establishing that you are a good person who would never do that, because reassurance is the compulsion, and the doubt would be back by the following morning needing more. What we do instead is work on how your brain responds to the thought, so that it can pass through the way other people's intrusive thoughts do.
I will also not judge you. You will not shock me. Helping people say the unsayable, and then helping them stop needing to fight it, is a large part of what I do.
If the only thing between you and getting help is the sentence you cannot imagine saying, there is more on how I treat intrusive thoughts and Pure O, and the first session does not require it.