I guess it's seen as not feminine by the judges?
And from what I've read, a lot of FBB's seem to think that aswell
... so just the ladies flexing in bikini's is fine for the whole family?
It's a weird double standard likely only enforced to discourage female bodybuilding in general (because it's not a moneymaker like bikini "competitions" are).
They want them to be symmetrical, but not huge. Cut, but not veiny or ripped. Have muscle control to flex, but not show it off. Have clear striations, but look feminine.
It's basically all designed to make it nearly impossible for anyone to know what they're looking for because they've wanted for years to dismantle female bodybuilding as a division.