The kink is called "hot past" if you want to read more about it and theoretically should be easier to digest than actual sharing content.
People talk about wild things they did in the past as part of getting to know someone (and it helps build that feeling of trust/being open)
Another important detail, in this case it wasn't an ex-boyfriend situation. More like a fling during her self-proclaimed "slut phase" - which honestly, she's still pretty tame all things considered. (more will be revealed in later updates)
So the FMC is actively choosing you, despite knowing what's out there - versus relying on innocence as the reason she's with you, as it is common in NTR.
This.
I get this kink, both irl and in games, and find it hot, not unnerving.
Shows the girl has adventure in her psyche, and may be willing to explore even further, with the right partner.
And then if she chooses to leave the relationship because I am somehow not "adventurous or capable enough", its my responsibility not hers, and life goes on, more fish etc....
Had a few women in my past (and present) who have this background.
People talk about "corrupting an innocent" as being somehow "the best"...
What is better? Corrupting someone even further than they already are. And knowing the elevated base from which you begin is a direct bonus.
These "past insights" can goad even better "challenges" to one's love-life explorations. A game of one-upmanship to see who breaks first, if you will.
Of course, if a boundary appears, a proper adult conversation can quickly sort that out, and thoughts can wander elsewhere, along other experiences. Life becomes much more adventurous and satisfying, because you know for a fact that the girl is already on-board for new stuff. Not some prissy back-water church-girl with a constantly shocked demeanour...just one with a cheeky grin as answer to new situations...now that is a hot look in a strange place...
And let's be honest, most "hot" girls get tempted into a "slut-phase" very early on in life, so if they choose me at some point, to hang with for a while, I take that as a good thing, not something to fret about. Enjoy the ride, I say.