Okaayyy I can't sleep so I'm going to talk about Siffrin's gender. Like a lot.
Siffrin is 100% canonically on the nonbinary gender spectrum. I, nor my husband (Do_Fish_Bite) disagree with this. This post serves to explain why our thought processes use masculine terms to refer to Siffrin. From my side and a bit on theirs.
What is important to note, and where I'll start this is Vaugarde canonically does not use labels, as stated in a anonymous ask Insertdisc5 answered.
Highlighted in yellow is the excerpt I find important to this conversation. I find it hard to believe theres no sexuality labels but there would be gender labels. Theres no labels outside the binary used in ISAT.
Naturally for Siffrin, he just feels. They don't have a specific word for how they feel. Which can lead to where you feel Siffrin falls on the spectrum exactly is based on how you interpret the text.
Let's talk about the two most popular pieces of text regarding Siffrin's gender.
It's easy to immediately read this as "Siffrin isn't a man" and just continue. But I would like to offer how I read this piece of text for your consideration.
Siffrin explicitly states not being a man off the bat but the rest of the thought isn't irrelevant. Personally, I find it interesting that Siffrin doesn't deny being handsome! It's the only adjective to "handsome, young men" that Siffrin doesn't question or reject. The line could've been (Not that you're a handsome man, but...) but it isn't. I personally take this as Siffrin being fine with being addressed by slightly more masculine terms, such as being considered handsome.
I want to immediately point out that this is not Siffrin speaking (obviously). Mirabelle and Odile would only know of Siffrin's gender based on what Siffrin has told them. And given the entirety of ISAT is based on Siffrin not opening up... Odile and Mirabelle wouldn't know how exactly Siffrin feels about his own gender. You can interpret that Siffrin told the party not to refer to them with any gendered terms beside their pronouns. But I am personally of the belief that Siffrin only mentioned being neither male or female, but didn't elaborate in detail what they feel like.
And the main thing is in these lines, Siffrin is stated to not be a man. It's about how they feel, but they don't go into detail about how Siffrin feels about the fine details about gender as a concept, the game never does. That's the beauty of insertdisc5's writing. So much of the story is based on how the player interprets the characters based on their own lives and world views. Some people view Siffrin as stiffly androgynous, and that's their view. Some people view Siffrin as being more comfortable with genderfluidity and different expressions of gender. Both interpretations are perfectly valid.
So where do you feel the need to correct other people on their interpretations of a character, when the creator themselves has said they enjoy seeing how other people interpret their work?
Do_Fish_Bite is my significant other, and they are agender. I myself am on the nonbinary gender spectrum also. We have talked endlessly about how we interpret the characters in private endlessly.
My partner uses they/them and is androgynous, but is perfectly fine with me using the maculine terms "husband, hubby, hubs" etc on them.
We reflect some of our relationship upon the characters we enjoy. This applies to Siffrin.
We both see Siffrin on the nonbinary spectrum, but is alright with gendered phrases being used on them here and there. Such as the terms guy, boy, dude, and even as a joke, babygirl.
Even insertdisc5 seems fine with the gendered term here and there.
This isn't erasure. It hardly even comes close to it. An artist on the internet interpreting a characters gender based on their own experience is hardly the issue you feel it is.
All you truly gain from doing this is just driving away artists who deeply love this series. Why would an artist want to post their art for your consumption if all you react to it is "Erm, actually!" behavior towards a joke caption?
Siffrin is on the nonbinary spectrum. Yet so is my own husband. Insertdisc5 doesn't mind different interpretations.
I've seen so many genderfluid, trans woman, trans male, agender, bigender, intersex, xenogender Siffrin interpretations. I personally think Siffrin is libramasculine!! It's a spectrum, and part of the fun of fandom is making your own interpretations and headcanons.
Don't treat others like this because you disagree with their interpretations and headcanons. Especially when they're well mannered and respect the canon material.
Just respect your fellow fans, because they just want to enjoy the game alongside you.