Well the rectum/anus is an interesting bit of construction. The initial chamber (rectum) is between 4-6 inches long and really not that wide.
The rectum does a sharp 9 degree turn into the Sigmoid colon, which does a J turn upward into the descending colon.
In fisting (you may have seen this in video) It takes a lot of practice and a lot of slow gentle movements to get one's arm up to the elbow - one literally shifts the internal organs around, straightening out the sigmoid colon, and even reaching as far up as the junction between the Transverse and Descending Colon.
If done properly and with a lot of 'training' - meaning many sessions of expanding and 'toy play' using larger and larger 'toys' to stretch open the rectum, anus and straighten out the colon - the human body is highly adaptable.
I'm using this as 'example' and strongly suggest you don't sit on a fist/arm or traffic cone any time soon.
Understand your ass is not designed to take things up it. It is designed as an exit-way only. The sphincter muscles are designed to push things out and to prevent stuff from going back up - stuff like water - which allows us to swim without taking on water and going down like the Titanic.
The sphincter and other related muscles like any other muscle can be trained, can be stretched can be exercised to do things outside of 'normal' activities. But like building up a large chest, or training your legs to be able to stretch to put your heels behind your head, it takes a lot of exercise, a lot of minor sessions where you slowly push your limits.
If you are bleeding from a (one) session of anal sex - you all went way too fast.
He is going to have to accept for a while that his wang is to large and he is going to have to learn how to to 'finger' you and slowly open you up. This means several minutes of slow probing and 'finger-fucking' with one finger, then graduating to two fingers then three - this can take a fairly long time - and hour wouldn't be out of the question.
We are talking about muscle - if you have regular sessions of sex say every other day type regular, a time will come - a couple three weeks, when you will not need to be fingered for an hour beforehand - you might actually get to the point that a little spit/lube and a slow sliding is all you need. Why? because your muscles will learned how to accept this invader.
Lube is all important here. The anus doesn't secrete sufficient amounts of lubricant to handle being fucked. Your anus does have a minor amount of lube, predominately the mucous that mix with your fecal matter - this makes it easier for everything to come out when you do your daily constitutional.
If you got to the point where you are bleeding so much you felt you had to bring it to Doctor's Attention, then its time to stop having sex until you stop bleeding and at least for 2 weeks there after to give your body all the time it needs to heal.
The most typical problems that lead to bleeding is torn or scraped tissues. Scraped is due to friction - tearing is often due to the sphincter being forced to open up way faster than it has learned to do.
You might want to consider using butt plugs - various sizes, you insert one for a few hours each day, starting with the smallest diameter ones. This will train you to accept the 'stretching'. -train your muscles to relax and accept. Graduating to thicker and thicker.
Dildoes and other toys can be part of the sex play, again graduating to thicker and thicker. Most sessions of fisting (the extreme stretching sport) start off with lots of fingering and toy play - yes even on guys who have years of being fisted under the belt need to be slowly opened up to accept thicker and thicker.