Think about WHY they're having sex. A lot of boring sex scenes are boring because everything was resolved before they started having sex. Resolve emotional or plot issues THROUGH the sex, not before. Even with a PWP where the "why" is "they're horny" then lean into it. Really empathize the desire, the need for touch, how hot they find their partner, the thrill they get from a particular act, whatever it is that makes sex more fun than a really good chocolate cake.