To be honest, I really don't understand why FGO fans are so desperate to ship their two bland self-insertion FGO protagonists with each of the heroines who already have a official canonical pairing in the franchise (e.g., Arturia, Jeanne, Nero and so on), although aware that these heroines will never be your waifu, nor from the two bland self-insertion FGO protagonists, or from any other mobage characters, because these heroines are already officially canonically paired with another character in the franchise.
And what you described is basically what fantasy doujins are about. Canon and lore be damn, they want to be the protagonist. Do those make sense lore wise? No. Do they care? No.
The sad fact is that most of these FGO fans blindly believe their own bullshit/fanwanks/headcanons, and also that these doujins and fanarts based on headcanon/fanwank/fanfiction are real and canon.
Either I'm missing something, or you have interacted with the wrong bunch of fans. Pretty sure any sane person, regardless is a fan of FGO or not, are very well aware fanwork are not canon in anyway.
To be honest all the servants fall for M/F Ritsuka anyway since they're *not* the same servants summoned from their parent series. Gotta remember, servants memories are reset after each summoning.
And the reasons they all love Ritsuka is how devoted the character is to helping them in their interludes.
I love a lot of things about Fate, however I do think a lot of Fate canon is all over the place and can sometimes or often be taken with a grain of salt. I have a friend that loves almost all things Fate way more than I do and even he cherry picks what lore he likes the most, which I think you do kind of have to pick your favorite parts of Fate and make that your own canon. It's interesting because this friend will sometimes consider things that Nasu didn't write as canon, he'll also regard somethings that Nasu did write as not being canon. So what's my point here? Fate is great, but to go on a rant about Fate canon on a piece of pornographic fanart because you don't like what people fantasize about, that's one of the truest definitions of absurdity I've seen.
Another thing to consider is our current age regarding multiple universes and such. Like it or not, there will always be fanworks, doujins, and projects by independent authors about the characters they like. Most of them treat these projects as alternate continuities anyway, and the Nasuverse is no stranger to this concept (hell, Prisma Illya started off as a fan doujin and look where it is now).
I think I can speak for a few like-minded folk that none of this removes/rewrites anything already established in the Fate Multiverse. Just shut up and let people do as they please.
Sieg... no one likes Sieg. He's sort of the one exception to the rule. Mainly because it is felt he stole the screentime in Apocrypha from characters with far more interesting potential since, well... he has literally no experience or history (as in, life history) and it shows. A not uncommon crack is that Jeane/Laeticia is/are in love with a cardboard box.
Ironically this argument can be applied to Ritsuka, Hakuno, and the majority of all other harem protagonists.
You can have a literal goddess who spent millenias upon an oath of chastity, but when she met this random loser who isn't even handsome/smart/strong/compassionate, she falls in love without the guy making a conscious effort, if any efforts at all.
Complaining Ritsuka/Hakuno is more justifiable than Sieg isn't quite a legit argument, considering Sieg(and maybe Caules) is the closest thing Apocrypha has to your generic self insert bland male who randomly got super awesome powers above everyone else, saves the day, and get all the girls(and traps) falling for them for no reason. He just happens to be in an anime/novel, therefore you don't have direct precedence over his actions and are less likely to think of yourself as him than if he is a game protag.
Regardless, FGO is a waifu/husbando collection game, saying fans shouldn't ship their self-insert with the characters is like against the point of the game itself.