Writing sex is really difficult. No, really. It is. (I can hear those of you at the back snickering. Stop it). If you've ever had a go at writing it, you'll know what I'm talking about. There's so much to get right, and so much that can go wrong, not to mention the fact that someone you know might read it and think that it's based on personal experience.
The first thing you need to do is to put the personal embarrassment bit to one side. If you're worried that your mother will read it, don't tell her you've written it. You don't have to show what you've written to anyone. Publish under a pseudonym. There are ways to get round this so that no-one will connect it to you.
So you've done that. You're ready. You're going to do it.
But before you start, there are a few things you need to think about. You see, when you add a sex scene to a story, it has to be about more than just the physical description of what goes where. Sex is not complicated (unless you're writing erotic alien menage, in which case you may need diagrams). The moves are simple and there aren't that many of them.
So what is important?
1. The sex must drive the plot. If it doesn't, it's gratuitous and there is no need for it. Even in erotica, each sex scene has to have an impact on your characters and drive their development and character arc. Ask yourself - what is the scene for?
2. The sex must make things worse for your characters (unless this is right at the end of the story, and the sex is part of the resolution, in which case your characters should have earned the right to a good time). Fundamentally, the characters have to believe that this is sex they should not be having. That doesn't mean it has to be illegal or kinky. Usually, it simply means that your characters feel that this is the wrong person to have sex with. That's what creates sexual tension - I want you but I can't/shouldn't have you - and so often why things fall apart and we stop watching after they've finally done the deed. Put consequences in place, make them personal and important, and tie them to your overarcing plot. Alternatively, you can make the hero Steve Trevor and make the sex wonderful but you will have to kill him off at the end (using the sex to make his death worse).
3. Think about language. It doesn't have to be filthy. You don't have to be particularly explicit if it doesn't work for your story/voice/style. Obviously different genres have different expectations so that will influence your vocabulary choices, but don't be afraid to put something explicit in your science fiction or something blurry and restrained in your romance, if that suits your characters. The character you have developed in the rest of the story must carry through into the sex scene. They can't have a complete personality change the second they take their pants off (and if they do, it has to be intentional, it has to make sense, and it has to have been foreshadowed earlier in the story). If you feel that you can't write the sort of sex this character would have, then fade to grey instead.
4. Think about what sort of sex these characters would have. Would it be angry/sad/silly/awkward/bad/embarrassing/disappointing/
overwhelming/surprising? Do the characters hate each other? Is this desperate, unrequited love or revenge sex? Would they be afraid of getting caught? Would they want to get caught? What has driven them to it? Why have they surrendered to their urges at that point? Going to bed with Tony Stark would not be the same as going to bed with Peter Quill. Once you've figured this out, you've got your basic moves.
5. Sex scenes that work well are about the feelings, not the body parts (see 4).
6. You don't have to describe every single thrust. A single paragraph can be more effective than an entire chapter, if it captures 5. See below.
7. The best way to learn how to write great sex is to read stuff written by those who do it best, and that's romance writers. (Don't pull a face! You can read romances on kindle, no-one will know). For sexual tension and sex that is open door, but not explicit, I would recommend Nora Roberts, Susan Elizabeth Phillips, Jennifer Crusie, Ilona Andrews, Lara Adrian. If you want the door completely taken off the hinges, Charlotte Stein and Kit Rocha.
8. Have fun.