Write the scene the way you want to, your agent will tell you if it's too graphic. But, think about your audience. Do they want or need graphic descriptions to understand what happened or how it effected you? If sex abuse is part of your memoir, you'll probably have a lot of survivors as readers. They know what being abused feels like, because they've been there. You don't want to alienate your readers with any implications that your abuse was somehow worse than theirs, nor do you want to turn them off. Then you've got the readers who haven't been abused- they can imagine what abuse is like, but they don't know it from personal experience. Do they want or need graphic descriptions? How does this progress your story? What are the potential benefits and risks to being graphic?
Sexual abuse is part of my memoir. I chose not to go into detail, because my story is not about the abuse but about the impact that abuse had in decisions I made as a young adult and the fall out from those decisions. I specifically did not want an abuse memoir, so I chose to only mention a few incidents to give the reader a window into what happened. My memoir focuses more on persevering in adulthood, coming to a realization things don't always happen the way they would if life was fair, so I did not need in depth descriptions of incidents.
Maybe your story hinges on specific details that happened to you, then you probably do need to be graphic. If the reader thinks you use gratuitous description you probably lose that reader.
