It seemed like a bigger leap to sexuality given the way Arya has acted and dressed over the years, and the fact that she looks younger than her age. Although Maisie is 22, I have a hard enough time getting to her screen age of 17 given those factors. Still, 17 is not young by Westeros standards.
I also agree with you from a career perspective. She has to move on with her acting life now. It will be hard enough as it is for her to get past this role.
There's probably an element of girl power in there as well..
She is the one who initiated it. It was her "choice." Perhaps a statement about women being sexual beings who can choose what to do with their bodies just as the men on the show can.