"Love" how desperate Maripologists are when it comes to excusing Marinette of keeping Adrien in the dark about everything, especially with how easily said excuses fall apart.
Putting my favorite excuses and the exact reasons they fall apart below, spoiler for Mr. Agreste for the last one.
"She didn't want to risk him getting akumatized!"
Cool except the fact that to her knowledge the butterfly miraculous was no longer an issue when she first told him the lies. She literally watched it fall into the water at the bottom of Gabriel's lair, hell given Time Tagger, she had every reason to believe in that moment the butterfly miraculous wouldn't resurface for a minimum of ten years thanks to the akuma being a teenage version of Chris as well as Alix being an adult. Meaning she has zero reason on not telling him the truth right then and there.
Heck, that's not even putting into the count that the butterfly miraculous has a range limit and how Marinette had possession of the horse miraculous again. In other words not only did Adrien have multiple hours to process information before the butterfly holder could even reach London, if Marinette had any concern, she could just teleport him to literally anywhere on the globe to allow him practically indefinite time to process everything.
Also, Adrien regardless on how he finds out, is going to get akumatized by this lie, arguably a thousand times worse now too due to how;
Ladybug willingly telling him the monster that is his abuser(who placed him in a torture chamber which she's well aware of) was actually a good guy.
Ladybug convinced the others in the know to do the same for the minimum of SEVERAL MONTHS NOW.
That's not even getting to the fact that the longer this lie gose on for the worse Adrien's reaction is going to be.
"It's not Marinette's responsibility!"
Marinette is the main character, thus it is her responsibility.
She is the superhero, it is literally in the job description.
Marinette literally made it her responsibility the moment she convinced all the others to keep it from him, alongside telling him the lie herself.
"Marinette is a teenager, practically a kid, She's not emotionally ready for this kind of talk"
See this could have been a decent and even justifiable reason… had the episode Revolution not claim that she and Chat are mentally mature enough to be considered adults.
Gabriel: How are they doing this?! Why aren't they transforming back, Nooroo?! Nooroo: You don't transform back either. Gabriel: I am an adult! Not transforming back is a power belonging to grown-ups! Nooroo: I guess they must have grown up, Master.
Meaning by the shows own logic, she is mature enough to handle this talk.
"B-but her telling him the true would make him hate her"
This seriously has to be one of the worst and downright weakest arguments anyone could possibly make. For starters, Adrien in all likelihood would have probably hated her more for the lie then the turth as the lie she told him makes her directly responsible for Gabriel's death. To make matters worse even if on the rare chance Adrien potentially hated ladybug had she told him from the start, he will absolutely and completely justifiably DESPISE Ladybug for keeping the turth from him for literal months, and for convincing all his loved ones within the know to keep him in the dark.
Hell the longer this lie goes on for the more justify his hatred would be, I genuinely would not blame him if he desired to cut ties with ladybug, and anyone else that willingly kept him in the dark! All of those involved in this secret, especially Marinette, have no one but themselves to blame for it.
"She was uh just respecting Gabriel's wish!"
Gabriel's last request was simply to have her remind Adrien of the times he tried to be good parent, not keep Adrien in the dark about everything. Heck given El Toro De Piedra, more specifically the letter we learned Gabriel made prior to the fight, there's now a decent possibility that he told her to do that so Adrien would find the letter in the box filled with his last good memories of him in order to convince Adrien to continue his fight for him.
Marinette had every right to deny him that request, not only due to the fact that Gabriel literally tricked and attacked her literal moments before making such a request, but due to how Marinette herself knows how much of an abusive piece of shit Gabriel was, I mean for crying out loud she literally found Adrien locked up in the isolation chamber Gabriel put him in!
"I-it uh isn't like the lie is h-hurting him!"
Marinette refusing to tell him the truth about his own biology has literally put his life at risk already with Werepapas, hell that's not even mentioning the fact that she literally played a game of russian roulette with his life in that same episode. Meanwhile the Gabriel lie? You can bet your name to the fae that is affecting Adrien! We literally see Ivan, the one person who could understand Adrien's own pain, unknowingly down play Adrien's own truma in El Toro De.
Ivan: It's hard for me….Your parents are role models for you. Your dad was even a hero Adrien! (Adrien looks away sadly) But my dad is anything but a hero.
Hell given the trailer as well as what was shown for Mr. Agreste at that one event a month or two ago( spoiler warning), we now know Adrien has to deal with people occasionally telling how great his father was, or that he should be grateful to had Gabriel as his dad.
In other words, Adrien now literally lives in a world where he can't even talk about the abuse his father put him through and now has to suffer in silence as a result.