First of all Mira seriously just ask him and I'm sure he would be happy to fuck you. Second now we know how he's going to win. It seems that she is literally weaker in the presence of men probably due to a curse from demons.
GOD, MIRA GET ON IT. Was there a explanation why Mira can't do it with Ex?
I mean there was the moment inside the shrine where she was denied but outside?
Besides getting cockblocked by elements out of Mira's control, I don't think there's an explanation given yet.
Personally, I think Mira is too shy/embarrassed to ask Ex directly for sex. That's why she tries to be coy in her attempts like the first time she entered the shrine with Ex and Yakumo.
GOD, MIRA GET ON IT. Was there a explanation why Mira can't do it with Ex?
I mean there was the moment inside the shrine where she was denied but outside?