might be a translation problem, not sure though
In Korea, like in many Asian cultures, calling someone by their first name only shows a very deep level of intimacy, like husband and wife or siblings. The reason why they suddenly went “in heat” is because showing that level of intimacy excited them. It happens to western cultures too, just with different words. It’s like when you really like someone and they whisper in your ear that you are gorgeous or that they love you. It can be very sexy. Hearing their name, in that context, pronounced in a certain way, while alone and half naked, probably excited them.
So why did they just get into heat when they called each others name? Weren’t they calling each other that in early chapters? I also notice that in k-dramas they use their last name not first name but for this didn’t they always use their names?