sex scenes

hello all.
i have seen this movie before, but upon watching it again with some few people.
I noticed there were some sex scenes, like a dream scene which wasn't bad.
but later on there is an actual sex scenes that seem to go on for a long time.

I just learn that Korean has rating called 18+ Rated, which would include strong gore or sex.
In fact majority of these types of movies are Adult Korean movies.

However this movie does not fall under the Adult Korean movie.

Is there a way to be notified on which types of movies has strong sex scenes?

thanks all