If anything it'll lighten the mood with a joke. They are dreading the conversation probably as much or more as you are and also probably won't make it a long one.

I dunno what specifically the content was but if you can pass it off as just artistic or art related I'd go that route. If it's clearly slappin material... You can always go the 'stumbled on it out of curiosity' route and then listen to the lecture.

Since you're 15, most sensible parents are, or should be, not surprised at this coming up at some point soon.

Hoping they aren't super religious, sexphobic, or bigoted people for your sake. If they are just regular ol' tolerant/accepting people then the anxiety will be worse than the actual discussion.