Ok, this is a short theory but actually it has many sense.... its about their hair
Obviously, you have noticed that when adrien transforms his hair slightly changes, well as he has never seen ladybug transform, he can easily think that the exact same thing happens to her, maybe thats why he stared at kagami on riposte episode(heartbreaking scene ;v) he could have thot that she could be the love of his life/ladybug/marinette
In the other hand, ladybug’s hair stays exactly the same as her citizen form, marinette, and again, she has never seen chat noir’s transformation, thats why she has the total right to think that chat noir’s messy hair could be the same without the magic.
This leading to a pair of blind teenagers and a very frustrated fandom
Upvote if you liked this theory and comment if you think it could be wrong! 😘🐱🐞❤️🖤