For years before I first had sex I wrote smut. Once I had sex for the first time, one of my first curiosities was to compare stuff I had written to the real deal. In the end, I had it pretty close just from high school sex ed classes, consultation of medical information when I had a question, and picking up information and fact-checking it from other stories. You don't need real life experience to write fiction, you just need to be reasonably well-read.

But, if you're determined for maximum realism, here are some things about real life sex to know. Some are, admittedly, opinions, and I unfortunately am only experienced with heterosexual sex. I also generalize here and there, and all generalizations are false.

A big thing porn ignores is that getting into position is not always fluid. It takes some time to get it right and it really helps if both people (or more people, whatever you're into) take time to figure out the angle and move together. If you just jam it in without considering the angle, it can be painful for both people. Whenever I attempt a weird new position I saw in a picture somewhere, more time is spent on logistics and "here, lift your other leg" than the actual sex.

It really helps if people are as flexible as possible, and unless both people do a lot of stretches there are a lot of positions that are practically impossible. Another big thing to consider with this is how tall both people are, especially if there's going to be any upright sex, such as against a wall. If the person penetrating is taller than the person being penetrated, they might have to boost them up, for instance, and even if you can carry your partner, it's really hard to hold them up while thrusting.

No one's a natural at sex, although that's really fun for stories. Males are going to be clueless around clitorises at first and females are going to be clueless around penises. Homosexual partners are probably at an advantage here, but penises and clitorises are like snowflakes, no two are the same and they're all stupidly complex. Having partners telling each other what to do or how to do it better is pretty realistic unless they've been together forever.

It can be really painful for a female to lose their virginity, though of course there are exceptions and some people feel nothing. Not everyone bleeds or experiences pain, but some do and for them it can really be a big fucking deal. Some people experience pain if they haven't had sex in a while and go back to it again too. Some people like to read this, others don't. Include in smut at your own discretion. It's also notable that there's more than one way to tear a hymen and a lot of gals who are athletic end up tearing them before ever having sex, especially girls who ride horses.

There are some weird sounds during sex. If you put your penis inside of someone, or put your hands or really anything inside of someone, there's a good chance it'll make an air-escaping-wet-space noise. In real life it's good for a grin and chuckle and you just keep going. Other people are into the sounds, your mileage may vary.

The noises people make with their mouths are not always sexy, and some people are just totally quiet. It's person by person. I used to think it was really stupid in porn when people are balls deep fucking and the girl doesn't say anything but "more" or "oh my god" or something like that and the guy can't even say anything, but now I realize that the main reason this happens is that it's really hard to get a coherent sentence out when you're having sex. Half of it is the motion/breathlessness, half of it is distraction. Single word commands and assorted pig sounds are about all I can manage.

Overall, sex is really funny! Between the strange sounds and positions, sometimes we just crack up. Not usually sexy to read, but good to include in smut if you're writing about a couple who have been together for a while. Some people get ticklish too and that's an extra can of worms to try to navigate when every time you touch them even a little bit they crack up.

The biggest perk of sex is emotional, not physical. I can fap and get the same physical happiness I do from sex, but I can't get the lovey-dovey feels. I recommend driving this point home when you write, a happy couple will be happier together after satisfying sex, and by the same token an unpleasant encounter is going to leave at least one person emotionally uncomfortable at the very least. 

People in relationships still look at porn and fantasize about other people. Porn is different than sex, period, and one isn't a good substitute for the other. They satisfy me in totally different ways, it's like the difference between liquid and solid food. I'd advise you against ever boning someone who doesn't want you to enjoy your porn and jerk off at your leisure but I'd also advise you never to put porn before your human partner. My partner and I share most of our porn even for things we aren't necessarily into. One thing early in our relationship that hooked us together was that we had the exact same /h/ picture of a couple cuddling in our favorite folders.

Real people have kinks, so it's not unreasonable for characters to have them as well. It doesn't hurt for it to end up in the narrative that so-and-so loves it when ladies wear thigh high socks if there's a gal with thigh high socks. If you have a recurring character you may want to flesh out their sexual preferences, like their favorite position and why they like it.

The big pattern among these points is that not everything that's realistic is sexy. And that's okay. You can leave out the unsexy things in favor of a sexier narrative nine times out of ten.

On an anatomical note, if you're gonna write you must absolutely familiarize yourself with human anatomy. If you didn't take sex ed, or if you did and you had a shitty school, go and read about it online. Learn the anatomy and the names of the parts. You should know this anyway for doctor's appointments in case you ever get sick and stuff (or you notice something on your partner's anatomy that you think would warrant a doctor's appointment). And please for the love of god remember that you cannot put shit inside a female's uterus without causing immense pain because it requires passing the cervix in the real world. Please do not use the word womb in smut unless you know what you're doing and/or have justified it with magic or aliens.