Ive taken dick and gave first time dick to some married guys and younger straight guys, and heres what I can tell you from my experience:
First, getting fucked is as much a mental process as it is phyical. You have to want it to start with and you need to learn how to relax your mind and body.
Your asshole has two ring muscles about 1-2 inches apart. An inner and an outer. The outer one is voluntary meaning you can control it and thats usually not a problem for getting fucked.
The inner one though is involuntary meaning you cannot control it, or do they say. Getting a dick past the first muscle is usually no problem, but the inner one can be a challenge. Your mind controls the inner one and when it senses pressure coming from the outside,it immediatly clenches shut as a defense response. Once it clenches shut it can be very difficult and painful to force it open.
Here is the easiest but foolproof way to fix this. WHen youre about to get fucked, try to concentrate in your mind and visualize those two ring muscle and imagine them opening paying attention to the inner one. if you visually can open it in your mind, your body will do the same..promise..just takes a little practice and you'll be the bottom whore you've always imagined yourself to be.