"He felt now that he was not simply close to her, but that he did not know where he ended and she began." -Leo Tolstoy's Anna Karenina
In Bed, The Kiss, 1892, Henri de Toulouse-Lautrec.
Every intimate moment has been painted or poeticized, lyricized, and immortalized. Writing intimate moments into your fiction depends upon many things, but here are some: the level of intimacy between your characters, your comfort level, what you promised your readers, and how well you know your characters.
From The Joy of Writing Sex: A Guide for Fiction Writers(©1996 Story Press), Elizabeth Benedict says:
A good sex scene is not always about good sex, but it is always an example of good writing.
The needs, impulses and histories of your characters should drive a sex scene.
The relationship your characters have to one another…should exert more influence on how you write their sexual encounter than should any anatomical details.
I don't write romance novels, but for me it's so important in my psychological thrillers that love is a strong element that drives the story as much as the murder mystery does. What's the point in spending so much time completing a book without a love theme?
Here's a site, rich in moments of literary intimacy: https://thejohnfox.com/sex-scenes-in-books/