found a deeply disturbing ‘dream journal’ about his sister in my bfs notes app from 2022. idk how to feel rn, is this a red flag ?

hey everyone, i really need some outside perspective because my head is completely spinning rn and i honestly don't know what to think. i’ve been with my boyfriend for 5 months, but about 3 months into our relationship i found a note in his phone from 2022. it literally starts with him saying he just woke up from a "weird dream" and wanted to write it down.

the dream he described is deeply disturbing and involves highly inappropriate, taboo, and incestuous themes with his sister. i cannot upload the picture of the note because i'm terrified he will find this post, but he basically detailed a dream where things got sexual between them before their dad walked upstairs and interrupted it.

when i found it, i confronted him about it. he looked completely disgusted, told me it made him want to throw up, and was totally disturbed by his own note from years ago. he was a lot younger back then and cringey, and his waking reaction tells me he hates that he even wrote it. but i just can't shake it. like is writing down a taboo dream like this evidence of some serious, sick underlying issue, or can the human brain just generate messed up dreams that mean absolutely nothing? should i look past this since it was just a dream from years ago and he’s disgusted by it, or is this a reason to leave?