An analogy I use a lot is this. If you go to the movies by yourself, it can be a wonderful and exciting experience. You get to choose the time, the place, the snacks, the row you sit in, and when you leave the movie you will have seen a movie. The movie may be what you hoped for or not, but you have seen the movie. (Barring random screen glitch failures.)
If you go to the movies with a person whose company you enjoy, there may well be some negotiations about time, place, snacks, and row. Throughout the movie, even if you never speak to each other, you will be aware that this other person is experiencing this movie with you. And at the end, you can talk about what you liked, what you didn’t, what was great, and honestly, even a movie you didn’t enjoy can be a lot of fun with someone who you enjoy.
If you go to the movies with a person you don’t know very well, there is negotiation which is often harder with someone you don’t know very well. And then once the movie is going, there may be things you don’t know. Do they like talking during movies or hate it? Do you like talking during movies or hate it? If you hate it, will they understand and shut up after you don’t answer a few times? Did they know to share the popcorn? Or did they keep forcing popcorn on you while you were trying to concentrate? And at the end, you don’t have any common experience to draw on to decide if they will love or hate the movie.
Any and all of the above experiences can be really fun! Or not. So, I think this analogy – because the pizza one wasn’t enough – also applies to sex. There are lots of internal thoughts, feelings, and concerns that your characters are dealing with in any of the above situations or any variations therein. (Also, think about going to the movies with people you know you don’t like. Automatically less fun. Boo.)
My plan is not to cover the technical physiological details of characters engaging in sex, but to talk about how the experiences and knowledge they have about each other and/or themselves will factor into their experience. This is also why in stories that are demonstrating a growing relationship, there can be multiple sexual encounters. Because by the second or third or seventh, depending on the story, there should be a material change in how these characters engage in sexual behavior with one another. They have learned new information about each other. They already know how the other feels about popcorn.