Marinette: I wanted to find out whether Adrien was ready to know the truth, but now I realize that I'm the one who's not ready to tell him. Tikki: Why would you say that, Marinette? Marinette: Because if the truth were to split up Adrien and me and I let myself get akumatized, the consequences would be catastrophic. (x)
And there you have it folks. Marinette outright says the reason she's lying has nothing to do with Adrien himself and everything with how she wouldn't be able to deal with his reaction.
Before anyone tries to claim she's saying this specifically because she got akumatized in this episode and so is traumatized by that, no she isn't. This had been a major, reoccurring element since the very beginning of the big lie.
Whenever we're shown what Marinette fears would happen if she told the truth, it's always about Adrien being angry with her and wanting to break up.
Marinette: I don't know how much longer I can do this! I lied to everyone, Luka! I told them that Gabriel Agreste was a hero, but the truth is he was the supervillain! He was Monarch! (Suddenly it's Adrien Marinette is talking to.) Adrien: What? My father was Monarch?! How could you hide this from me Marinette?! You lied to me!! You and I are over!!! (x)
In fact, this was a fear she had since even before the finale. It had nothing to do with Gabriel's words or actions.
Marinette: Adrien! I'm going to save you! (A large dragon appears and attacks Marinette with a gust of flames. The girl fights back and defeats it. The dragon turns out to be Gabriel Agreste akumatized. He detransforms and falls onto the vine, unconscious.) Marinette: Mr. Agreste? (Trips.) WOAH! (Falls over and accidentally touches Gabriel with her sword, making him disintegrate.) Adrien: Father, NO! (crying) Why did you do that, Marinette? (x)
Following that, whenever she talked about why she's lying to Adrien the reason she gave is that Adrien will be hurt.
Rena Rouge: But why lie to Adrien?! Ladybug: Because the truth is too harsh. (x)
That's the only reason she ever gave.
Luka: Why did you [lie to Adrien]?! Viperbug: Because I love Adrien more than anything, and he'd be in so much pain if he'd ever have found out! He could get akumatized by the new holder of the Butterfly Miraculous. And I'll never, ever, EVER let anyone hurt him! (x)
Every. Single. Time.
Marinette: But if I tell Adrien the truth it'll do him too much harm, and I can't bear to see him suffer, Tikki. Tikki: And I can't bear to see you suffer like this. Marinette: Either I suffer by lying to him and Adrien remains happy, or I tell him the truth and we'll both be very unhappy. (x)
And you'll note how the problem with the lie Tikki brings up doesn't have anything to do with the morality of the entire thing or Adrien's well being. No, it's all about how the big lie is emotionally burdening Marinette.
That too, is a repeating topic.
Marinette: If you want, I can cancel my liar lesson—I MEAN—guitar lesson! Adrien: No, no. Go ahead. Say "hi" to Luka for me. (Marinette begins to leave, but once she sees Nathalie she runs back up to her.) Marinette: I can't take it anymore! We have to tell Adrien the truth! (x)
Marinette doesn't bring up any other reasons for why she nearly broke there. It was mere that she "couldn't take" lying to Adrien anymore.
The only time anyone ever brought up how the lie was hurting Adrien was Alya in Revalator. Every other time we see Marinette conflicted over the big lie it's solely a matter of how the lie makes Marinette feel bad and hurts her.
Dream!Luka: Lying, lying again, always lying. The burden is too much to bear. Dream!Rena Rouge: You'll feel better once you're rid of your secrets, Marinette. Let's drop the masks. (...) Marinette: Those weren't my friends, Kaalki, they were just projections of things I wish for. (x)
Even Nathalie realized that the only reason Marinette wanted to reveal the truth to Adrien is because keeping it a secret made her feel bad.
Nathalie: You need to grin and bare it, Marinette. It's too late to go back! (x)
"Grin and bear it" is what you say in response to someone complaining that something sucks. Marinette has no other arguments for why they should stop lying to Adrien other than how she, personally, hates lying to him.
Marinette: Ugh, I hate lying to Adrien! (x)
But just because she hates lying to him isn't any indication she thinks it's morally wrong. It's not even an indication that she's thinking about how it affects Adrien.
Ladybug: Cat Noir, do you know what it's like to go so far, almost too far in order to protect someone you love? Cat Noir: You mean like sacrificing oneself? Ladybug: More like... say... keeping a huge secret? A secret so heavy that you really wanna share right this minute because it's too much and you feel like crying and you just need someone to comfort you? (x)
Marinette's conflict about the big lie is always, ALWAYS, brought up in the context of how bad it's making her feel. She never ONCE thought or said anything about how the lie might be hurting Adrien. In fact, she's fully convinced that the lie is for Adrien's benefit, despite all evidence to the contrary.
You might be tempted to think that the reason Marinette feels bad is because she knows lying is wrong. But luckily, Marinette cleared that up for us as well.
Marinette: Dear diary, that's it! Adrien and I are officially a couple. Even when I feel his hand in mine, I cannot believe it sometimes. But I think I know why. The truth is, I still can't be myself because I lied to him and to the entire world and- (x)
Marinette is not a girl that can cope with difficult things by herself. You only need to look how she survived all of two episodes being the guardian before breaking and telling everything to Alya for proof.
Yet this is not something she can do with the big lie. The lie is too big and everyone she turns to criticizes her for it rather than comforts her the way she wants them to. Heartfixer shows us how much the big lie troubles her thoughts, many times more than everything else she'd been through combined. The result is that she's more stressed than she'd ever been in a way she doesn't know how to handle.
There have been a lot of claims in fandom about why Marinette is lying. So let's do a recap based purely on the things canon tells us.
The reason Marinette is lying to Adrien is because she thinks the truth will hurt Adrien and that he'll blame her for hurting him.
The reason she hates lying is because she can't ask other people to comfort her and alleviate her stress without revealing the truth.
Let's stop pretending it's because of anything else.
