***Disclaimer***
I'm about to drop some hard facts brass tacks, if you choose to move forward then you accept all responsibility for stepping on said facts/tacks and hurting your internet feelers.
Definition of 'slutty'
1. dirty; untidy; slovenly. 2. (esp of a woman) sexually promiscuous.
I'd like to put forward that it doesn't rule men out of this.
Hard facts: They jumped each other in the toilet of an airplane faster than a fat kid would raid a candy store after the apocalypse. That's both easy and slutty. Hell, even the context of the situation and conversations say as much. Random sex, never thought to meet again. That's text book slutty right there. What we lack is history of previous actions. Is this a one off? Or is it typical behavior? Story gives somewhat conflicting information. They both act slutty, (the toilet on the plane)but at the same time she hasn't jumped on any other D but his. She's definitely easy though. But she's also been faithful to the guy that she seems to now have feelings for. You can't call her behavior since the plane anything but easy, she's definitely been putting out without any kind of relationship, but not been a slut since the plane. But the MC kinda is a manslut, lol.
The problem here is that people on both sides have started infusing their opinions and social beliefs into the facts and on others.
You can view her behavior as slutty, or you can view it as socially acceptable "easy", but by definition she was still a slut on that plane. Just hasn't been one since.
So, to close, her behavior on the plane was promiscuous behavior, hence slutty, but her behavior since the plane has been rather faithful to a single partner, so not promiscuous, hence not slutty. We can debate till blue in the face, won't change anything. So maybe we can lay this to rest and agree to not agree amongst each other and just enjoy the story? Cause'...that damn ending yo'.