What makes you think Krem is part of the Qun? He's a human not a Qunari, not to mention that whilst Iron Bull is technically a Qunari, an integral part of his character is that he has problems with the way of the Qun, so it could be that this is part of that reason. If you haven't played the game, it's slightly unfair of you to suggest that there is no lore justification. I'm sure there is one in the game.
I am assuming Gaider will create an in-lore justification, but like the others mentioned, the Qun seems to be pretty clear about people having to accept their assigned role in life.
Here’s a piece of dialog from Dragon Age: Origins :
Sten: Why are you here?
Leliana: What do you mean?
Sten: Women are priests, artisans, farmers or shopkeepers. None of them have any place in fighting.
Leliana: I have no idea how to answer this...
Sten: It is not done. There is no more to it.
Leliana: Do you mean your people have no female mages or warriors?
Sten: Of course not. Why would our women wish to be men?
Leliana: What are you talking about? They don't wish to be men.
Sten: They shouldn't. That can only lead to frustration.
Leliana: Sten...no, never mind. Let's drop this.
That Krem isn’t the first transgender character in Dragon Age by the way. There was a transgender prostitute in Dragon Age II (admittedly a very minor NPC), and another transgender woman in the comics - Varric’s cousin’s wife, if I remember correctly.
But hey, popular media. I remember a quote from ‘300’- ah, here’s the one:
‘
Leonidas: Submission. Well, that's a bit of a problem. See, rumor has it that the Athenians have already turned you down. And if those philosophers and... boy-lovers have found that kind of nerve—‘.
What super-manly Leonidas didn’t mention is that the real Spartans were boy-lovers
par excellence, but hey, that doesn’t sell in a product intended for a mass audience. If 300 can rewrite actual real-world history, then Gaider can alter his own lore if he wants to and put in all the transgender pansexual Qunari he wants.