Layman, a premarital affair depicted in film can consist of more than a sex scene and may not include a sex scene at all. In older films sex scenes are depicted just in their foreplay (kissing and hugging) and then the screen fades before any clothes are taken off.
Regardless, FWIW, the Vatican’s list of recommended films includes films with nudity and even with sex scenes.
Not surprisingly, even though given the thumbs up by the Vatican and only a PG rating from the MPAA, the USCCB has given it an A-III rating. Sex in films is not nearly as potentially morally corrosive as violence is. For example, take the very entertaining and worthwhile film Wanted. IMO, the most morally problematic aspect is not in any sex scene, nor in any gore (there’s really not much of either), but in a potentially ambiguous message it might send to vulnerable children who haven’t yet been properly formed in their ethical education.
Anyway, in the Vatican’s list, it includes The Mission which includes a fairly graphic (read: intense) sex scene (even though the movie is rated PG … probably today it’d be re-rated PG-13 due to more stringent standards when it comes to sex and nudity).
If you are really concerned about these kinds of issues, use this website:
It is very detailed and also unlike almost every other service (it’s free) mentions whether God’s name is taken in vain. For ex. for Punisher: War Zone, it says in the Language section:
“3 religious profanities, 3 religious exclamations.”
At first they didn’t include such information but I guess from patron feedback, they decided to include it.