No, that's the thing. Even if we pretend it's a female garment (even American "kimonos" aren't), I really don't see why using the word in a turn of phrase would be "sexist".

It's not implying in any way that women are prone to flashing, that women would be in any way inferior, or that actual women should open their actual kimonos. You are just using the image of a (supposedly) female garment.

Just like saying "like a kick in the balls" isn't sexist. It doesn't imply you want to kick men or anything like that.