Before we jump into how you write a good sex scene, let's first talk about the different ways that sex can be written in books.
In MY opinion, there are 3 ways that you can write about sex in books: you can be super detailed about it, you can be super PG about it, or you can be a little clean and a little dirty.
LET'S BREAK IT ALL DOWN!
the detailed/graphic way :: when it comes to books with this writing style, I'm reminded of one-shots. One-shots are short stories that are quick and dirty and to the point. I'm not talking about all one-shots, but rather, those that specifically fall under the erotica umbrella. These stories don't always have to be super raunchy or smutastic, but it's kinda the point to have it that way when you're writing those types of stories and there's nothing wrong with that.
Now, when it comes to being super detailed and/or graphic while writing sex scenes, for one thing, the author doesn't normally shy away from calling body parts exactly what they are: a dick, a cock, a pussy, nipples, etc. instead of choosing safer works like shaft, member, etc.
I'd like to say that this style is all about the details and descriptions.
Some authors will go all out and will literally break down the sex that's happening; for example, she watched as he lowered his head and settled in between her legs, and then she felt it, the flick of his tongue against her... You get the picture lol and the whole scene will go like that which is fine for some people, but SOMETIMES those types of super detailed scenes can be emotionless and flat.
Long story short, this style of writing focuses more on what's going on vs how the characters are handling things mentally and emotionally.
For obvious reasons (underage users!!), Wattpad and most sites similar aren't too welcoming when it comes to 'smut' or hardcore erotica, but if your book is telling an actual story and has a plot, then you should be safe even when writing detailed/explicit sex...
Just to give you a better explanation of what's allowed on Wattpad, per their rules:
the clean/the PG13 way :: this type has sex scenes that are 'clean' and kinda basic, not too detailed or too graphic and normally doesn't contain much or any dirty talk.