i guess she thought mc was female when they were children?
Well, she uses a male pronoun for him when they were kids, but, assuming it is '-kun', that pronoun is also used with children sometimes regardless of gender, so it's unclear.
Regardless, she was much younger then, so maybe she had some negative events with men occur between then and now.
I'll be reading this as it updates, I'm sure. I hope this isn't one of those bait-and-switch gender bender stories though where the ending is a reversal of the gender bending to make the relationship het again, since I'm here for the Yuri. The author is sort of signaling this will be Yuri though by naming the MC Yuuri.