Just to be clear, ace people can enjoy sex (although there are plenty out there that find the idea repulsive). Being asexual doesn't mean your nervous system stops before it reaches your genitals, it means you don't have a libido. Ace people will still have sex with a partner that they love and trust in order to make them happy, but it's not something they actively want to do. Asexuality isn't sexual dysfunction, it's sexual disinterest.
At least, it used to be.
But then it became a new and exotic hat to wear for people who both wanted to be part of the exclusive cool kids club that is the LGBT community without actually having to be gay, but they also didn't actually want to be asexual so they came up with the 'gray ace' spectrum, where you're suddenly considered part of the 'spectrum' if, say, you only want to have sex with somebody you've emotionally bonded with. You'd think the super-simple binary of 'I want to have sex' and 'I don't want to have sex' would be easy to understand, but people really want to identify as something rare and unusual to elevate their boring lives instead of finding a way to enrich themselves so might as well just open that umbrella until its inside-out.
To bring this back to Lily, she has said (and isn't alone in saying) that 'straight people are boring'. In her Korra review she explicitly says that Korra baffles her because Korra is bisexual and yet in spite of her sexuality is boring. She has legitimately made the argument that her trite romances are elevated beyond their station just because lesbians are involved.
Very minor fairness here: people do like comfort food. They will read the same story over and over again if it's something they enjoy. The romance genre as a whole is basically this but swapping out the players depending on what you're interested in, and there authors that churn out the same story over and over again and make mad bank just by doing that. There was an author my parents were reading back when I was in high school who had the exact same formula for every one of his books (I mean, beat for beat), and you know... sometimes it's fun to just go on a dumb adventure or escape with a different dashing romantic hero, even if it's fundamentally the same story.
That said, Lily doesn't really write stories, she writes scenes that are the same two characters having the same conversation over and over again with slightly different set dressing. There are no plot beats to follow because there is no plot. TSR only has a plot because she's patterning it the stories off the movies, and those stories cleave so close to the movies that even the changes she does introduce are just cosmetic. I totally get liking to explore particular scenarios and conversations because they're what interest you specifically, but that's all Lily does. Just those scenes. And then publishes them and expects endless praise.
The other problem is that the definition of Mary Sue is so broad and has become so warped that it's hard to know what shorthand people are using. Generally speaking, people that use the accusation of 'Mary Sue' are using it in its classical context, meaning a character who has bent the world to their will and faces no true adversity because of it. The people who are accused of writing Mary Sues, meanwhile, use its diluted definition -- which has been specifically formatted to deflect criticism -- of saying that people just don't like strong female characters winning.
That's why she chose the examples she chose-- the characters involved are not just considered to be overpowered, but they're also male. She's saying "I bet you like these characters because they're MEN and only dislike these other because they're WOMEN", which is not only a presumptive argument but false. I used to spend a lot of time in litcrit circles in high school and college, and the books that were chosen for criticism were almost entirely male-dominated and male-focused, with the common complaint being that the MALE PROTAGONIST was overpowered, had the universe bend to his will, encountered little opposition, and was overall boring and flawless. These kinds of books also generally had female leads who were little more than love interests. Everybody involved called them out on both aspects-- nobody likes a Gary Stu, and nobody likes a useless love interest.
This is an extremely common criticism of bad books in general, but the difference is, when you're criticizing male characters, you're just criticizing characters. When you're start criticizing female characters, well, obviously it's because you're just sexist. You have a whole stack of books and movies you've called out for having shitty Gary Stu protagonists? Well obviously the only reason you didn't like Captain Marvel is because you're a misogynist. I best you would like Rey if she were a man.