This will be a long reply, I hope you have time!

When it comes to hate sex your characters should save a cat!
I know it sounds weird, but bear with me, it'll be a long answer but it'll make sense!
The concept of "saving the cat" comes from "Save the cat" by Blake Snyder - which is a book about screenwriting, but very helpful for writers who want to develop more complicated stories! I really recommend it if you want to write a long fic but are scared because thinking of creating a plot makes you anxious.
So what is "saving a cat"? Basically, whenever you have a character who is doing terrible things and should be hated, you need to give people a reason that will make them sympathize with them and root for them! Robin Hood is robbing people, which is generally considered a bad thing, but he's giving the money to the poor, so that makes him likeable. If he was robbing the poor too, for his own benefit, we would hate the guy, wouldn't we? The bully in a high school movie is treating the nerd horribly, but then we see a scene where he takes care of a stray dog and is incredibly gentle with it, so for sure he has to have a reason to treat the nerd so badly, right? The protagonist is a rude, moody guy with no social skills, but we need to make the readers like him so they finish the book. So what do we do?
We make him save a cat in one of the first scenes. And now they have a reason to root for him.
But how does it connect to hate sex?
Well. Your characters need a reason to want each other too. Maybe they're on the opposite sides of the conflict, but they admire each other's devotion to the cause or intelligence? They definitely at least have to find each other attractive - why would you write sex scene between two main characters if they don't lust after each other?
The conflict between them makes writing the tension easier. There's already energy between them, you just need to redirect it. Physical fights were used as a symbol of sex for ages (if you want to read more about it, I can't recommend "How To Read Literature Like A Professor" by Thomas C. Foster enough - chapter 15 "It's All About Sex..." from 3rd edition has more examples of unsexy things that symbolize sex!) and are pretty similar in writing to smut - you have to consider the limbs all the same, there's plenty of thrusting and throwing and shoving, a little more blood and a little less other fluids (unless you add both, which - hot.) The sounds are already the same - moaning, grunting, groaning, whimpering... All you need to add is more sexual tension and a little less desire to kill each other. And when it comes to verbal fights, they're already a perfect setting for flirting. Nothing more agitating and sexy than a well placed condescending praise like "aren't you a smart boy?" or "I guess you're more than just a pretty face, hm?" after your enemy makes the right move. Charge the air! We want tension so thick that you can slice it with a knife and one spark away from everything blowing up!
A lot of the time, when the characters are physically attracted to each other, it's the rivarly, the way they challenge each other, make each other grow what makes them want the other person. It's easy to find someone hot. But when they get you, it's what makes the relationship more exciting, even when you're on opposite teams. Do you think they would be friends/lovers if they were on the same side of the conflict? The answer should be yes. If they wouldn't you don't have enough ground to make them lovers with the conflict added.

As for angst - it depends what is causing it. I love stories with exes who want one last night together, especially if one of them (or even better - both) is still in love, yearning and clinging to the last moments with their lover, greedily taking as much as they can, pouring their love into sex, prolonging it because they don't want it to end. Bonus points if they start crying during it. Hot.
Or you can have something bad happen to one of the characters and they either initiate sex to forget, or their partner initiates to make them forget. Just don't do it if the source of angst is sexual assault, that would be distasteful. Or do, if you want to make the angst even worse. It's up to you.

In any case, you need to consider what's the point of the sex scene. What are the characters trying to achieve through it? Resolve the emotional tension - whether it's anger mixed with lust or sadness? Show dominance over each other? Make up? Remember that written sex is never about sex, and if you write sex and mean only sex then it's simply porn! (and that's another quote from "How To Read...", chapter 16 this time "...except sex" seriously, this book is great!)(also there's nothing wrong with writing porn itself, but I think you wouldn't have trouble with making it realistic if you were there just for porn, hm?)

Oof, that was a long answer (and I just had a heart attack because I am literally at the end of it and misclicked and I thought I lost all of it), and I probably skipped some stuff, but I hope it's helpful enough!