It's not an easy one, because the way people generally talk about sex in English wants to fall into one of three options, depending which words you use; crude vernacular; anatomically clinical; or somewhere lost in the forests of overblown metaphor. Steering between these is difficult indeed.
A lot depends on what you are actually trying to do, too. I'm of the opinion that a sex scene, like any other scene, should be there for a specific purpose; ideally more than one. And, well, if what you are trying to write is "A works for B, and B suspects A's loyalties are flagging, and B is sleeping with A thinking it will bind them more closely, and A's loyalties actually were not flagging but A is kind of insulted by B thinking this will help and so B is achieving the exact opposite of what B is aiming to do", so much of making that work is focused inside people's heads and on what they say that the "insert body part X in orifice Y" is not really the foreground of the scene. (I have nothing against sex writing that exists to titillate the reader, I just rarely find it works well with characters whose in-world motivations I am connected to.)
