One of the things I've noticed in your writing is that Amy usually receives versus gives oral sex to Dan and its vice verse for him and I'm interested as to why you personally believe that's how it would go in those types of situations.
Side point, I do not like writing sex scenes. I always have Austen’s advice to her niece ringing in my head: “You describe a sweet place, but your descriptions are often more minute than will be liked. You give too many particulars of right hand and left.”
Anyway, there are a number of different reasons for this, which sometimes interlock. However, the biggest one is simple realism – like many women, Amy cannot always orgasm through penetrative sex (or at least, cannot orgasm in the short time the penetration lasts), so half the reason Dan gives more than he receives (orally and digitally) is to even the balance, and make sure she gets as much out of it as he does. I know fanfiction is often a fantasy, but I think the idea that penetrative sex must lead to orgasm can be quite damaging for women who, you know, need a little bit more than that to get there, so I’ve tried to maintain what I consider a realistic balance where sometimes it’s enough for Amy, and sometimes it isn’t, and when it isn’t Dan does something else to get her off. In other words, it’s a “from each according to their ability, to each according to their need” scenario.
I’d also argue that this is partly a reflection of Dan’s character as well – as I said previously “he has the common sense to realise that Amy will be more interested in sex with him if she enjoys herself,” hence he makes an effort to ensure she does. The lengthy scene in 6.05 where he talks about maybe having sex with Jane certainly sounds like he’s describing oral sex (I think he mentions his face potentially being melted?), and I’m certain he’d put more effort in with Amy than Jane. In fact, given how much he likes to tease Amy and make her lose her composure, it seems likely to me that he’d find being able to make her lose control quite arousing.
There are also a couple of occasions where Dan gives oral sex because he’s trying to make a point. Like the scene in Chapter 9 of You Were Always On My Mind – think about it, he’s told Amy that he stayed in DC to be around her and Emily, they’ve finally rekindled a sexual relationship, she’s staying with him in the beautiful house he bought for her, and she claims not to understand why he’s making an effort. How annoying would that be? So giving oral sex in that particular scene is a way for Dan to emphasise that he is doing it for her, everything is for her (and no, Amy doesn’t understand it that way, but that’s what he’s attempting to convey).
Similarly, in the chapter of What Might Have Been where Amy’s father dies, there’s a distinct calculation underpinning Dan’s actions. Because his fear that if they have penetrative sex Amy will hate him for it isn’t irrational – she’s very, very vulnerable in that scene, and if she ever thought that Dan had used that vulnerability for his own gratification…their relationship is already on unsteady ground, but his fear is that it would poison things between them permanently. So he gives without receiving because there’s less room for misinterpretation that way – even in her most emotionally raw state, Amy can’t perceive that act as Dan taking advantage of her, because technically he doesn’t get anything out of it. (Even so, she still runs like hell as soon as it’s over, because it opens a door to increased intimacy with Dan at a time when she is hurting more maybe than she ever has before – and he’s not the safest person to be around).
Finally, I would argue that because of the way the show has perpetuated a power imbalance between them over the last two seasons, with Amy apparently having unrequited feelings as far as she knows, it’s made it more difficult to write a scene where Amy gives without receiving that doesn’t feed into that imbalance somehow. Not impossible, but more difficult.