It starts because Chat remembers Ladybug's voicemail asking if Marinette was "cute" way back with Evillustrator and snowballs with every dodged request for Multimouse on the grounds of "keeping her safe" since her "identity was known".
And Chat, as he's want to do, puts 2+2 together and gets fucking 23.
Now Chat thinks Ladybug is not only in love with Marinette, but has been secretly pretending to be in love with the famous oTheR BOy maybe to throw Chat off her scent, or maybe because she thinks he won't accept her, oh my god does she think he's homophobic??? What should he do???
Be a good friend and support her, that's what!
Meanwhile, Ladybug has been going through the 5-stages of grief thinking Chat has been scorned by Adrien Agreste at some point since every time she mentions the model Chat's response is increasingly hostile. At first she thought Chat hated Adrien, but all his comments seem to be critical of his inability to face conflict?? Or show his true feelings?? "Pretty boy" is said with such scorn that Ladybug (who is also the Anxiety-Catastrophic-inclining Marinette who regularly declares her own death at minor inconveniences and whenever she gets hit with her long list of secondhand embarrassments) has made up a whole backstory of how Chat is secretly bisexual and is using Ladybug to get over his horrific attempt at trying to express his feelings towards Adrien oh poor Chat she totally understands where you're coming from you poor kitty.
Cue Chat trying to wheedle out Ladybug's secret relationship with Marinette by making more comments about Multimous and how nice it was to do that mission with Marinette! Can he do it again? While also making more visits to her balcony to figure out why LB fell in love with his shy, awkward, yet brilliant classmate and oh god oh no she's actually really funny and sassy and way too cool for Chat or Adrien LB has fucking TASTE!!
(he also has been really emphasising his support for gay rights every time he hangs out with Ladybug (which isn't helping his 'secret pining bi' angle LB is frothing over) to the point where even local news outlets and the LadyBlog are debating whether Chat is queer af (which has always been a pretty firm belief in Paris, what with the leather outfit and hand gestures like ✨you know✨) or just pro-LGBT+)
On the other hand, Marinette has being trying to suss out this ""secret mean side"" of Adrien's that has made Chat into such a scorned lover, resulting in her being a little cooler to Adrien, which in turn has made her actually capable of speaking to him now he isn't some perfect figure on a pedestal.
("Plagg! Marinette frowned at me when I accidentally bumped into her today and told me to 'watch it, pretty boy' before leaving!!! She didn't stutter like usual, which I thought she did because she hated me, but what does this mean now????"
"That I'm not paid enough for this")
Except Chat has also been sniffing around her balcony a lot and she can't help but feel sorry for a fellow Adrien-Lover, so now she's been spending a lot of free nights with him and wow he's actually so sweet and shy beneath the flirty attitude and oh no he's actually kind of hot, and if Ladybug was the rebound from Adrien does that make Marinette a viable love interest??? W H A T???
This got away from me, but honestly we're at the point that it'd be so much funnier to make the lovesquare even more complex like yeah fuck it up you two dumbasses.