Technically the human anus is not really designed to accept a penis sliding in and out of it. By design its pretty much meant to hold things in until you push things out. Its an evacuation canal.
However owing to the flexibility of the materials it is composed of, that muscle can give a bit and that the anus connected to the colon is all pretty much soft tissues that are flexible and to a degree stretchable, one can manage to insert rather large objects, penises, fists, traffic cones....
Key to accepting minor and major objects is relaxation and initial bearing down in order to cause the sphincter and anus to 'pucker' outward. Over time and lots of lube and slow movement one can eventually stretch out the anus to take most anything you can think of and most likely even things you can't think of (but someone, somewhere most likely has a video of that).
Some guys (and I assume gals) are able to accept a lot of 'stuff' going in. I'm uncertain if this is a design flaw or a design advantage. I guess it depends on who you talk to. On the other hand there are people who are just not able to accept anything. And no amount of coaxing or patient experimentation leads to their being able to take it.
Perhaps these are just qualities of a tight ass?
This illustration gives a good idea of the canals, the bit just above the external anal sphincter (the rectum) is about 4-6 inches in length in the typical human adult male. If your BF is of greater than average length (Average being something on the order of 6.25 inches) it is possible that he can readily 'bottom out' hitting the back (upper) wall where the rectum gives way to the Sigmoid Colon.
Now I'm sure you have seen images of guys taking a fist and the forearm all the way up to the elbow (if not I strongly suggest you do NOT look for such on the internet). The fist and forearm are typically much, much longer than the rectum, what happens is that through patient manipulation and perhaps a bit of fairy magic, the sigmoid colon and rectum and be moved about to align and lead to a straight passage.
I point this out to you to demonstrate just how flexible the human colon is.
If your partner is impatient even with an 'average sized' penis, he can do damage and could conceivably hit bottom especially if you are more compact in the guts area, thus having a shorter rectum.
The diameter of the rectum ranges between 1 to 1.75 inches, being more commonly 1.5 inches in diameter. The diameter of the average penis is roughly the same. But if your partner is endowed with a bit more than average, it is possible that if you are blessed with lower than average guts that it (his penis) is just too fat.
In the 90s lots of polls were taken about the Gay community (Straight people are fascinated by gay stuff in a creepy way) any way, the poll revealed that unlike what the porn industry of the time kept pushing as being 'average' gay sex (you know, a bit of oral leading to anal for the climatic big finish), the reality is that the majority of gay men actually don't like anal, they prefer oral. Most do anal because they thought that that is what is expected, since their education of what gays do is based primarily on what straight (usually bigots) always think about, anal sex.
It appears that many gay men really don't get into anal, most finding it 'uncomfortable' when they bottom to the point that they would rather not do it at all. And a goodly number of them go 'ew, gross - poo on a penis - ew!' just like many of the bigots who think about such things all the time always say (but hey we all know they find the idea to be fascinating, thus their preoccupation with what they think gay men do).
The point is that you do not
have to do anal sex. Chances are your partner is doing it because he thinks that that is what he is supposed to do (Gay men are known for tossing salads, packing fudge and other back door activities - so it must be so).
Chances are no matter how much coaxing and techniques you try the sheer idea of having 'that huge thing' shoved up your bum may not lead to your being able to relax enough to accept it. Its ok, you don't have to do it.
you can still be gay in other ways... like oral sex.