Cocks are different, asses are different, sex situations are different. But the basic reason many beginners experience pain is basic anatomy.
You have not one, but two sphincter muscles in your ass. The outer one is the one you are most familiar with - it is under your control. You can tighten it to keep from pooping in your pants and you can relax it when you finally make it to the toilet to let everything go.
However, there is another muscle an inch or so away at the base of the colon that really does the work of keeping shit in so you don't have to think about it all the time. That's why to don't shit the bed when you fall asleep.
Of course, this whole system is designed to keep stuff from coming out. The challenge is to retrain it to let something in.
The outer muscle isn't much of a problem - you can relax it. It's the inner muscle that's tricky. Because your conscious brain can't tell it to relax. It's kind of like if you hold tension in your neck or back - the muscles tighten, but you can't directly relax them. You have to learn how to do that indirectly.
This can be hard, but you can learn to relax your whole body and create the conditions where that inner muscle can also relax. Of course sex is surrounded by emotions that are anything but relaxing - horniness, fear, performance anxiety are just a few of the demons that can rise up and make relaxing that inner muscle even harder than if you were just at it on your own.
Old joke: How do you get to Carnegie Hall? Answer: Practice, practice, practice!
The best way to learn how to relax so you can bottom easily is to practice on your own with toys. Like in a nice warm bath, or some time you set aside for yourself where you can make yourself feel comfortable and take your time. Without outside pressure, you can learn the keys that allow your colon to open and let in an object. Once you know where those feelings are, you can add a partner.
People jump in thinking that nature will just take her course and fucking will come easily. It's not that easy. But you can learn and be ready.
And its also very important that tops listen to their partners and follow their cues.
For a good top, fucking is always about maximizing the pleasure of the bottom, not just "getting off". If you, as a bottom feel that you are not being listened to and respected, get out of bed and turn on the lights and either have a conversation about what you need or throw the motherfucker out. You deserve better.
