You nailed it right on point about those two muscles.
They just need practice to relax comfortably for anal sex.
The Bottom guy needs to take that initiative with
Cleanliness and Preparation.

By practice, I mean dildos and butt plugs to get used to
being anally penetrated. A relaxed bottom can handle it
with ease after a good preparation for it.
The top also needs to check his own brute strength
while penetrating the bottom.

Anal sex should feel enjoyable, not so painful that it
leaves you unable to shit straight for several weeks.

I've often shook my head in mockery at all the so-called
"anal virgins" who'd gone on Craigslist seeking to get
fucked by the biggest dick possible when they have NO
darn clue how to prepare their tight asses for the
penetration, probing, and pounding that awaited them.
Many of them never think to WASH THAT ASS before
getting fucked, and the results are gross!

I once fucked a Bottom guy who Didn't clean out,
and was on a full stomach. Halfway through pounding him
I was covered in SO Much shyit that I looked like I was
mud-wrestling a beached whale!
I dragged his fat 240lb dumb ass into the shower
with the quickness.

I was like "WTF?"... what is so 'difficult' about
cleaning one's own ass out with a water enema?

Dear Dirty Bottoms: Google for info about How to
clean your ass with an enema, PLEASE!

Anyways, back on topic...
A Very Tight ass can often be frustrating if it
won't relax to allow a cock inside...
...wanna piss of a Top? show up with a frog-tight ass,
and he will kick you right out the door.
I've had very few guys whom I've wanted to kick
da shyit outta them for wasting my time with their
frog-tight asses.

So, all you so-called "anal virgins" out there in
fantasy-land take heed: Do The Research, Douche Out,
Stretch Wide (butt plugs and dildo preparation),
and Loosen Up BEFORE you go to meet your Top guy.