Ah, the reason is simple, you would have a hard time keeping her in check. Unless you're into getting your girl fucked by random guys, you shouldn't avoid settling down with someone like her.
And it goes to all women you interact, the number of guys they fucked etc, matters a LOT. The number matters.
So, if you're planning to wife someone, firstly don't and if you really do think she is the one, make sure you explore her completely before giving away the wife card.