most of the aruani smut fics that i read usually have the armin eat out annie scene and i love it but can you explain why this is kind of a headcanon?
I think it comes from the what the act represents in Aruani sex rather than the act itself. I have to elaborate a bit on this so bear with me :>
There are many ways where you can write nasty smut, keep it purely porn-without-feelings and have it work perfectly well within canon/au/divergence/whatever. In general that is, it's very much possible to strike a balance with smut that doesn't need much characterization or an insight into too many emotions, yet remains highly enjoyable.
It's possible to do this with Aruani smut too (and many authors do it well!) but also - it's difficult.
As characters, so much of what makes Armin and Annie the people they are is how they feel about things, places, people, others, each other, and so on. These are characters who have been developed within a very complex world under very difficult circumstances, and therefore have more unsaid about them than said at any given point of time. I believe as a couple, even with the canon moments we have, the way they behave around/with each other says a lot more than any of the words they speak. Not to mention, the ways in which Armin and Annie contrast each other adds more complexity to their interactions. To give a very simple example, Armin leans toward selflessness, while Annie leans toward selfishness. These are foundational pillars on which Aruani operate even in a very plain environment, making it a difficult task to strip them away while keeping them true to character and behaviour.
Now, sex being a very intimate and emotionally vulnerable act in and of itself, it goes without saying that an Aruani smut scene will have, in some way or other, a display of their barest, most naked and raw feelings. They are emotional characters even outside of a sex scene, so stripped of their clothes and courage they make for a very vulnerable, insecure, frightened pair. Here again, you have their contrasts at the forefront: Annie's impatience, Armin's hesitance; Annie's eagerness, Armin's consideration; Annie's desire to have it all at once, Armin's determination to take his time... these add flavour to the kind of sex they might have. These are character points that spill over into the act of sex - you see why it would be so hard to take it away?
Finally, coming to the question at hand - eating out is hot xD Agreed, everybody agrees, it's like the question appearing on the exam every year for the last 10 years lol. But when authors write Aruani and cunnilingus together, it's more than the, let's say, eroticism of the act alone. Armin doesn't go down on Annie just cuz it's the mandatory bit of sex, it's because he likes to give, he likes to please Annie, he likes to watch her rise higher and higher in ecstasy while he postpones his own high - actually we can even say this is also his high - it's a spillover of his selflessness, he's going to give Annie everything.
And she's on the receiving end, of course, and doesn't complain (okay lets be real lol, she can't. she won't xD) - it's like a symbol of Annie's selfishness, her taking what he gives, accepting it because it feels good, because it's addicting, because it's a rush and she wants to keep climbing it. It's also a symbol of Annie, finally after years of being deprived of so much, accepts happiness and love and pleasure from an act on the most vulnerable part of her.
In short, when we write the eating out scene, it's (more or less) an subconscious way of adding characterization to Aruani. A maybe-maybe not unintentional added depth into the type of people they are under the pressure of sex. So Armin eating Annie out isn't just eating out. It's a tiny window into who they are in a naked, intimate moment.