That was a new take on the super-annoying trope of "you unknowingly proposed to me, and I am aware you couldnt have known my culture, but you must love me considering that, so because of this I love you too! Now you must get married and cant decline, as me saying this is just a response and you can neither change your mind nor argue that the intent behind a proposal matters."!
I always wanted to see it get shot down as utter silliness some time, and not just have character go with it, but that will never happen. This is rather close to subverting it though: "I thought you were male, so what you are saying is that I am gay and proposed to a male. That or the custom of yours only applies if you were female, making the 'proposal' invalid and turning you into a male, bro."
edit: At least this way there is no chance of the manga going harem route, as the fmc class-changed into the arcetype male friend (aka background character). There is only one other fmc left apart from the main heroine, so it will either be a love triangle or he will manage to class-change her as well to turn it into a vanilla romance.