I did not really mind things when I first watched the anime, and still don't to this day. However, I understand that there are things some people (if not most) saw as toxic (whatever it means). To me, it was simply a fictional romance that was written the way it was need, for good or ill.
[Start]
Ohgi helped Villetta in order to get some information about Zero. However, let's say that instead of taking her home, he brought her to an hospital and after that kept visiting her to check up on her.
Still suffering from amnesia, as I doubt Britannia as a cure for it, she hasn't told anyone about Zero. Eventually, she's discharged from the hospital, but due to her condition she's removed from active duty for the time being and offered to be sent back to the homeland for recovery.
However, she choose to stay in Area 11 and desire to repay Ohgi for helping her and visiting her at the hospital. The few people in the military that knew her before the amnesia would definitely find that out of character for her to grateful to a number, but let it slide since it stem from her being amnesiac.
Eventually, she invites herself to live at Ohgi's place, and things starts to follow the canon progression (Homemade bentos with octopus wieners and the Ashford Academy Culture Festival "date", etc.)
[End]
This is just a base idea and is by no mean full proof. I'm sure some people can points out some flaws. In any case, how would you picture a better written romance or, at least, less "questionable" romance for these two?
Also, for the people that are not pro OhgiXVilletta or simply have a low opinion on the topic. This post isn't about "How toxic/problematic this relationship is", but "What if it was written better, how would it be written?" So please, stay on topic. (Maybe I could rephrase that last paragraph better, but I'm not sure how. In any case, I do not wish to antagonize anyone. We're all Code Geass fans and have are own opinion.)