I think there are several exceptions to this. Sex scenes in general follow “show, don’t tell:” having a character simply do X is usually preferable to having a character say “I did X.”

Some examples off the top of my head:

• The Terminator - It shows how the main character becomes pregnant with minimal exposition
• Shoot ‘Em Up - It highlights the absurdity of the film, equates sex with gratuitous violence, and tells you about the characters
• Sausage Party - it is an illustration of The Absurd in the wake of an existential crisis
• Team America: World Police - It functions as shock humor that also satirizes other gratuitous sex scenes
• Irreversible- There is simply no amount of dialogue that could illustrate the amount of horror and violence the scene conveys and bring about the proper emotional response from the audience
• Jackie Brown - It shows how pathetic the main characters are
• Tomcats - It creates a comedic moment when the timid librarian turns out to be a dominatrix
• Trainspotting - The film compares the high from heroin to an orgasm, and the sex scene extends that metaphor and helps the audience empathize with the main character’s struggle
• Requiem for a Dream - Shows how Jennifer Connelly’s character has hit rock bottom