Writing good, convincing sex dialogue is way harder than I expected!
Because there no such things as sex dialogue, and therefore no way to make it convincing ; at least convincing for everyone. There's talk, sometimes dirty, during sex, which is different.
Think about what happen for you during sex, remember how it was one year ago, there's no real constant. Remember how it was with your previous partners, there's no real common points. It's just things said during sex. You can record your own sex session, then transcribe it, that even for you it would not feel convincing enough. Yet it was the dialogue the last time you had sex.
I remember when I was still a teenager, one of the girl I was with was both shy and kinky. She wanted (relatively speaking) dirty talk, but was too shy, or perhaps not confident enough, to say things like "fuck me". So she made up words to replace the ones she wasn't able to say. It was something like 30 years ago, so don't remember the words ; perhaps that in fact there weren't effective words, and she was just saying whatever sound to replace them, using a different one every time. Imagine hearing things like, "you zumbaze me so hard", "hmmm, your yumb is so deep", and others, "fuck my etmir". There's no way for someone to see this as convincing sex dialogue, it's too ridiculous. Yet, she's surely not the only girl like that, and there's people around the world who would, like me, feel it as believable.
Then there's the girls who only moaned, while others were talking a lot, sometimes too much. Some people internalize everything, others are at the opposite and say everything single things out loud. I even had one who, according to her, was so sensible that she had to say everything that crossed her mind in order to not finish too fast ; a thing that guys know well, except that we keep it in our mind.
In a way, sex dialogues are the same thing than drunk talk. We don't know what we are saying, nor care about it. We just feel the moment, and when we say something it's just random things that cross our mind and are related to the instant. But there's no coherence, nor effective constancy, and what was said once can perfectly not been said a second time.
Since I'm on the memory lane, one thing I remember was my wife once saying, "oh yes, fuck your lovely doe, you fucking hunter"... then us having to stop because we laughed so hard at this stupidity. She was picturing herself as the poor innocent victim of some bad guy, and that's what came to her mind at this moment. We don't think what we'll say, we just say it, and sometimes it can fall totally out of place.
Another similarity with drunk talk is that it depend of the context. You'll not say the same kind of things when you're lightly drunk, than when you're deep bottom. And it's the same for sex dialogues. A girl that usually just say things like, "harder", "faster", but never really more than one word, never really dirty things, can time to time say things like, "oh yes, break my fucking bitchy ass", because this time everything was over the top, her mood was right, she was hornier than usual, and all ; the timing was perfect and, her pleasure being over the top, she lost all inhibition.
I talked about the action, but obviously the same apply for the preliminaries and all. In the end, whatever how hard you'll try, you'll never achieve to write convincing sex dialogues. The only thing you can do is write ones that looks believable enough.