Before you charge ahead and try to get his cock jammed up your butthole, it'll help you both if both of you know a little about the anatomy of the ass.
Compared to a woman's vagina, a butthole is much less suited to penetration. It doesn't self-lubricate, it's made of less sturdy stuff, and as many people who don't like the idea of anal sex will quip: "It's an exit only hole."
Obviously, it was primarily designed for expulsion but that doesn't mean you can't hack it for a sexually positive perverted pleasure. You do need to take care though.
Natural bodily mis-functions can cause a tear or do damage to your anal canal. Things such as:
A large bowel movement
Hardened stool (as a result of constipation)
As a result of even these minor traumas, you may see blood on your toilet paper. So you should know that for your delicate dixie-hole, even gentle anal sex can be traumatic.
Enjoy anal sex and avoid injuring this delicate area of your body by going slow, always using good lube and always, always stopping when you need to.
When you look at your anus... ahhh, not that flexible? Get a mirror. Ok, so when you look at your anus in the mirror, you'll see a beautiful chocolate starfish. Or yours might be another flavor. Human skin tones vary dramatically so expect yours to look a little different to anyone else's. Unless you've found your anus doppelganger? Wohoo! Anyway, that tinted, puckered area of skin covers the outer anal sphincter.
Touching, stroking, massaging, or licking the anus is great stimulation. The same way someone touching your armpits can feel ticklish or make you pull away defensively, you may feel the same with your anus but when you are relaxed and comfortable, it can evoke a positive sexual response.
However, this area of skin is sensitive so if stroking with a finger, adding some saliva or lube is a good way to get a smoother glide, otherwise your finger may irritate rather than stimulate.
Under the skin of your anus, as mentioned, is the inner sphincter and also the outer or external anal sphincter. The outer is thicker and surrounds the inner. Both of these muscles surround and make up what is your anal canal.
If you insert your finger, your anal canal is the part that grips your finger. The grip comes from the sphincter muscles and after the first or second knuckle, where the grip relaxes, you have passed through the canal and entered the rectum.
When you aren't thinking about it, the outer muscle is in a state of partial clench - https://en.wikipedia.org/wiki/External_anal_sphincter.
You have some control over the outer muscle. Clench your butthole and it's that sphincter you are squeezing. This is conscious control - similar to moving your hand. It does what you tell it to do.
However, poke it and it will react autonomically and clench shut. But you can relax to touch and learn to control the outer sphincter.
When it comes to anal sex, your internal sphincter is the problem. You have no conscious control over the inner sphincter and it is always fully clenched. - https://en.wikipedia.org/wiki/Internal_anal_sphincter
Your internal anal sphincter doesn't want you to have anal sex. But you can beat it simply by knowing how it works.
After you eat, the food is digested and makes its way down through your guts and when it's finally released into your rectum, it's simply having something there waiting to come out that makes your internal sphincter relax. When that happens, your outer sphincter kicks in and you may even have to consciously clench. Your ass may even start sweating! It's time to poop so your internal sphincter is taking a break and the external doesn't like the hard work.
Initial anal pain and injury occurs when you force this more delicate internal sphincter open when it doesn't want to.
The trick to pain free anal sex lies in tricking the inner muscle to relax by first slipping something thin into your rectum, triggering the inner sphincter to relax and from there, it's simply a matter of being relaxed enough to unclench the outer sphincter.
Passing the anal canal, you enter the rectum.
The job of the rectum is to tell you when you need to poop. Normally, poop will stay further up in your colon until it's ready to be pushed out. (Hey, can't talk about anal sex without mentioning poop.)
When it drops down into your rectum, you can feel it. You get that feeling and you know it's time to go. So when you get a cock or dildo in there, initially you are going to feel like you need to poop. That's natural, normal and though quite disconcerting, it passes. That's one really good reason why you need to get comfortable with how things feel up your butt by yourself.
If you continue up through your rectum you get to your colon (also known as the large intestine). At the end of your rectum, there's a sharp bend where is turns into the colon or sigmoid colon https://en.wikipedia.org/wiki/Sigmoid_colon (meaning the lower part of the colon).
This sharp curve (more than 90°) means when something like a very long penis gets to the end of your rectum, it feels like it's hitting a dead end. Trying to go further than this is not advised.
Part of a Whole
From your lips to your anus is a complex, continuous system that works together with a universe of bacteria. In fact, without all these other living organisms, we'd be quite unwell.
You might be worried about what could come out of our ass during anal and consider internal cleansing. However, your body's digestion and consequently your comfort relies on all these organisms living in balance, and internal cleansing can be disruptive to this balance.
Cleanliness is important but you can go too far. The balance lies in being clean but still respecting your ass as part of living system; in enjoying anal sex but still knowing it's not sanitary and respecting this fact by enjoying anal safely.