sorry this took me a while to get to - i had to think for a bit about this one. also sorry it's super long. also thanks to @rmd-writes for letting me bounce some ideas and giving me your own!
i've come to the conclusion that i think writing sexual tension goes far beyond the actual sex scene/intimacy scene itself. i think the thing that makes sexual tension feel believable is the build up - every interaction the characters have prior to the actual 'getting together' scene. it's in their dialogue, it's in their thoughts when they stray to each other, it's in the little things they accidentally let slip in conversations to side characters. those are the things that really help culminate in a very sexy scene.
utilise dialogue and non-verbal cues. i know this is a very cheap shot because i'm a lover of dialogue in general, but characters who feel a type of way about each other are going to act differently together than they do around other side characters. try to build in character-appropriate dialogue, for example: awkward, nervous conversation OR confident flirting OR suggestive, leading sentences (this will depend on the characters, obviously). you can do this both in WHAT the characters say, and in HOW they say it. I.e. dialogue tags can be really helpful here. if a character is flirting, perhaps they wink. if they're awkward, maybe they blush. maybe their sentences trail off. maybe they hold each other's gaze for a fraction too long. maybe they lean in closer? go wild. don't forget the small details too - you want the characters to notice things about each other. the colour of eyes, the curve of their lips, the way their fingers curl around a glass etc. FURTHER, i would add that dialogue in sex scenes is (in my biased opinion) very sexy, and a great way to keep the scene moving. just the right amount of 'you like that, baby?' or 'just like that' or lingering eye contact with a 'sweetheart' on a breath etc. etc. can not only make the scene 10 times hotter but also work in great communication.
don't rush. this goes with the dialogue aspect, but letting sexual tension build is really important and depends on where the characters start (enemies to lovers is generally going to take longer than a PWP one night stand). Over multiple interactions, the characters can start to appreciate different things about each other, which lends itself to showing the reader attraction. This can even work in short fics. For example, if two characters meet at a bar, perhaps you can work in a scene which cuts through multiple hours - one character looking at the other and noticing how they look on the dance floor, then laughing with their friends, then skimming their fingers over a glass at the bar, then flirting with the bartender. In each of those instances, the observing character can notice something new and more intense about the character in question. it builds believability and tension.
in sex scenes, use sensory details. aside from the use of dialogue in sex scenes, another thing which i think is incredibly useful in building tension is paying attention to the senses. describing what a character hears: the frantic beating of a heartbeat, a gasp, laboured breathing. the feeling of their fingertips on the other characters skin, how their hands feel threaded through the other's hair. the smell of the other character's cologne/perfume or the salty taste of sweat on their skin. all of these things are super important, build tension and further add to the sexiness of a scene.
stay in character. i think it's important when building tension to make sure the characterisation stays consistent. you do want the characters to ultimately be attracted to the real version of each other. as comfort levels grow, so might flirty banter, sarcastic or witty comments, laughing with each other, friendly touching. depending on the type of story, i would aim to build attraction by having the characters slowly reveal parts of themselves to the other, because it adds to the growing togetherness, but also keeps them consistent.
i hope these are helpful! thanks for sending me this ask, i love answering asks like this!
