It's never been said anywhere that all the dragons are male, not by anyone who is in a position to know that, actually, nothing has been said on their six one way or another. Now, I'm sure that in lore, there are female dragons, but will we see distinctions between the sixes? I don't know, we will see. Admitably, it might not be immediately obvious just by looking at them, but that doesn't mean we can't know their six, they could be distinguished by signs other than the obvious physical ones we'd use to determine the six of humans or other mammals, like how in Morrowind even though female Argonians lacked any feminine mammal traits, you could still tell they were female even if you didn't hear them speak (Their voices were naturally female.) Maybe female dragons have different colors from males, or maybe they're physique is different, maybe they have different shaped horns, smaller horns, or no horns, there are a lot of design choices you could make to allow us to tell the difference between the sixes other than the obvious ones, you'd just have to give us some indication that the differences represent different sixes first since otherwise one might not be able to tell.
Though I suspect that the only way to actually distinguish the sixes of dragons is to hear them speak or hear them referred to as either male or female, assuming we see female ones at all, I wouldn't be surprised if all dragons speak in your stereotypical deep masculine voice, and will be adressed as male if it's necessary to refer to them by any term that might indicate six.
They are called Jills.
I think they are all female.
I don't recall Bethesda ever said the dragons in Skyrim are Jills, one might speculate they are since they're dragons serving Alduin who is apparently also Akatosh, but that doesn't necessarily mean that's the case, seeing as Skyrim already seems to have revealed some things about dragons that aren't the same as what we previously accepted as true about them, it may be that this aspect has been changed too, or maybe Akatosh/Alduin has other servents besides Jills, who can say at this time?