I am a bisexual man who can give you helpful information and even share some stories that may help provide some techniques that will allow you to accept a cock up your ass so that it will pleasurable for both of you. Remember before you engage in any type of anal sex, you need to be safe and responsible.
Preparation
There are some things that need to happen to prepare yourself and your partner for anal sex. Step one is cleanliness. You always need to make sure your asshole and rectum are clean and prepared to handle dick. If needed, use an enema about an hour before engaging in anal sex and then clean yourself really good after you have expelled the enema liquid. Now your asshole is prepared to accept a guy's manhood. This cleanliness also allows you more options about where to take his spunk. If he pulls out and his dick is still clean, then he can put it against your face and give you a facial or shove it in your mouth and shoot his wad.
Step two in receiving anal sex is properly preparing your partner's penis. I do not have to tell you that size definitely matters. The size of your partner's member will determine the type of condom he will wear if needed, type of lubrication needed, amount of lubrication needed, and the difficulty it takes to fit it in you. If you just look at your partner's dick, sometimes that can be very deceptive because first it may not be completely hard and second, you will judge it according to the proportion of his body size.
For example, if a large man is going to fuck you, his cock may not seem as big as it actually is when comparing it to the size of his body. This is the reason why I always suck a guy's dick before I allow him to fuck me. By going down on your man, you first will get him completely erect, get him hornier, and you can feel his complete size with your mouth. Now that you have his cock completely engorged, you can make better decisions on proceeding forward.
Condom size is very important. You want to be sure you have a supply of magnum condoms as well as regular size. I like to put the condom on the guy myself to guarantee proper placement, fit, and to guarantee the reservoir tip is directly over the hole to catch all his cum. I once hooked up with this black man named Randy who I met at a bar and we ended up back at my place. He was very well hung and neither of us had any large condoms so we just used a regular sized one I had in my drawer. I should have listened to my instincts when I had trouble getting it on him. Eventually, he was holding my ankles while furiously pounding me in the missionary position and when he unloaded his spunk, I thought I could feel the warmth of it while he was shooting. When he pulled out, I was horrified to see the condom was completely broken and the only thing left on his dick was the rubber ring around the base of his shaft next to his pubic hair. Randy had completely shattered his rubber because of the size of his dick and the friction of my tight asshole.
I have also had the complete opposite happen to me. The first time I allowed this waiter named Tim to fuck me; I put a large condom on him because he had a long penis. Even though he was about nine inches in length, he was very thin in girth. He fucked me for quite a while in several different positions and because of the duration and him leaking so much precum, his rubber completely slipped off inside me. Once again, we didn't know it until after he came. It took a couple of minutes after we had finished for me to find it and pull it out.
Whether you are going to use a condom or going bareback, you need to use lubrication. Most of the time, you want to use a water soluble lube like Astro-gluide or KY Jelly. These types of lubrications will not interact with your latex material and are easily expelled. Occasionally it is nice to mix it up and use baby oil, Vaseline or something creative. I once was at this guy's apartment that had just moved in and most things were boxed up. I was giving him a blowjob on his couch when he got so horny and needed to fuck. We looked around in boxes for anything to use and the only thing we could find was peanut cooking oil. Surprisingly it worked very well and we were able to kill two birds with one stone because he ended up pulling out and cumming in my mouth and that mixed with the oil taste was not bad compared to when it is mixed with regular lubrication.
I find it best to lubricate both, his cock and your own asshole. There is really no reason for both of you to get lube on your hands so decide which one of you will do it. Most of the time, I am the one who greases up my partner's dong with some stroking and then I slather my own asshole by sticking my fingers in and swirling them around. I personally like to do it myself because one, I can control the amount of lube and second, it prevents him from having it all over his hands and he won't accidently smear it all over your hips, ankles, or waist when he is holding on for leverage.
Semen is a great lubrication. Of course it can only be used if your partner has the stamina to fuck you twice or if you're involved in a threesome or an orgy. One time I was involved in a threesome where both guys came twice. After the first guy blew his load in my ass, it was lubed well enough to last the total sex session with both men. After taking 4 loads in my butt, I did feel as though I had a cum enema and not once was there any trouble getting them in and out of me.
Entering
Once you are clean and both of you are lubricated properly, it is time to fit him inside you. This is the hardest part. If a guy has a large dick or even just a large head, it can start out very painful. Your lover has to be very careful and patient. If he has a smaller to medium sized cock, I will allow him to insert it my rectum but if he is extremely large or just has bulk, I like to control the entrance. If I am unsure, I will error on the size of large.
If your lover is the one that will be doing the inserting, it helps if you reach back and guide him. Place his penis head up against your hole, take a deep breath, and relax your sphincter muscles. When he pushes, keep your breathing smooth and controlled while you allow the mushroom to pop in you. Your anus will stretch but it still is quite painful in the beginning. Try to hold it as long as you can and if the pain gets too unbearable, have him take it out. At this time is when many people give up on anal sex but you have to be persistent. The second or third time he slides his head in your butthole, the pain will be a lot less. Your hole will eventually adapt and accommodate his size but it takes time. This is why I say your partner has to be patient. This is another reason I like to be in control if my partner is hung.
Once he is inside you, and the pain has subsided, then allow him to feed his inches into you slowly. This can also be painful and this is why he needs to be patient and well lubricated. I prefer he goes ahead and buries his cock all the way in me until his pubic hair is touching my ass crack as soon as he can because this allows you to get acclimated to all of it and there are no surprises later on. Once you are ready, he can begin his thrusting.
Now if your partner has a large dick, you have to approach it differently because he can do some serious damage to you and this is why I like to be in control. After I have sucked a guy to full erection, I will have him either lie on the floor or sit on the edge of a chair so that I still have access to put my feet on a hard surface. I will hold his hard cock where it is pointed straight in the air, squat down on top of it, place it directly against my asshole, then ease myself down on it until it is tolerable. Sometimes, it may take as many as three or four times of you pulling off and then getting back on it. Eventually, I will be sitting on his lap with his huge dong buried in my rectum.
If you're properly lubed, stretched, and able to take his size, then you can have him fuck you. I like to bounce on him for a while just to get everything adjusted and then switch to different positions of his choice. You can always finish by going back to the original position of riding him until he cums. I will tell you that a large dick can make a difference inside you. When your butt is completely stuffed with a huge dick, the length of it feels like it is up in your intestines and the suction can put air in your stomach and give you cramps. When fucking a well hung man, you need to take his cock out of you periodically during your sex session. Don't wait too long before you get it back in so that your hole is still stretched and ready.