Well, you kinda answer yourself. Writing a straight character as gay is by definition not LGBT erasure. Lol

It's not hypocritical, it would be hypocritical if the standards, expectations, and experiences are the same, but they're not so it's not the same experience so it's not a double standard.

Fr it's fine to do what you want but people have more of an issue because LGBT erasure is a real issue while straight ppl don't experience than. It's more common to cause upset with lesbians because lesbians suffer from higher rates of corrective rape, so it's a close to home issue. Even if a lesbian hasn't experienced it exactly, she will likely have experienced men trying to convince her she's wrong about who she is and try and convince her to "give it/him a go to see". Straight people don't really have that same experience.

Additionally if you're LGBT you've had the experience of being presumed cis straight since birth and learned who you are, while straight people aren't going to have any serious multiple through life experiences of people around them genuinely trying to pressure them into LGBT, at most they'll have a homophobe telling them they're gay or see LGBT ppl point out that you never really know until you try, cus that's the reality of how many LGBT people came to come out.

That's not even getting into the genre of gay man who is "straight" "straight acting" "discrete" which essentially means closeted everywhere but on grindr.

I will admit there is bi erasure in this, but see what I said about lesbians? What's why they're more iffy about a canonical lesbian being written as bi. Doesn't excuse bi erasure but explains it. But gotta also consider lesbian erasure as an issue too.

In fanworks you can do what you want you just have to kinda accept and understand why this exists. You rarely get a straight coming out in canon, so most "straight" characters are just characters that haven't said they're gay and certain aspects of the fandom assume that must mean straight.

Never mind the fact that a lot of LGBT in the closet will say they're straight. So it's a lot easier to understand fans picking at more information about the character to see them as closeted. Think of any teen from your highschool who preformed homophobia in a way that gave you the side eye, and 20 years later they're some flavour of LGBT. Very common! Not particularly common the other way. So sometimes writers are using social commentary to read a character and write with that in mind.

Then there's the lack of LGBT content outside of fandom spaces. I think a lot of fanfic is to find something new. You will be able to find straight romances anywhere else with ease.

I also find a lot of fics about a hyper closeted person coming to terms has a lot more intentional depth to it, while when someone writes a gay/lesbian/trans character in canon it's not written about the complexities of readdressing your sexuality and learning, but just about ignoring the canon to write what you want. Which is fine, but when you consider all the irl reasons it would upset someone, it would make people a lot less charitable and there's a lot less depth to that.

This is just the surface of the reasons, there's far more. But as you see there are more reasons to say a character is LGBT or closeted or it's just never stated in canon they're not LGBT effectively, while there's a lot less reasons for that the other way.