I hope it was your movie and not a rental. I really don’t have an answer for you if it’s a mortal sin or not. I wouldn’t say that it is, but whether it’s morally appropriate for you to watch is something I think you have to decide for yourself.
I remember hearing about this movie when it came out years ago-- I haven’t seen it, but I just read the plot at IMDB and now remember recall what it was all about.
I’m sure the US Conference of Bishops must have rated this movie back then and probably gave it an objectionable or adult sort of rating. You can always read their reviews before you watch a movie.
To commit a mortal sin you must have knowingly known it was a sinful act you were participating in before you actually did it. You didn’t know anything about this movie beforehand, so if you stopped watching it, when you felt it was sinful, I’d think you’d be ok. Another thought, by watching it, is it likely going to make you commit a mortal sin? Then I’d say, yes, it probably was/is sinful.
Honestly, I don’t know if this was a sin or not. I’m inclined to say no-- but am curious about what others think.