Yeah, but she's been running all this time with people who don't give any credit to that stuff. Remember when Nynaeve, Elayne and Birgitte had to play nice on the boat ride, lest they give their misogynist captain the satisfaction of fighting, and then ended up becoming sincere about their friendliness? I think the same thing happened with Berelain. She played nice with Perrin while his wife was gone, and it stopped being playing after a while. Her discussions with Faile and her new insight into their own way of looking at relationships probably opened her eyes some more. After all, given her description of her sexual experience, her outlook at that point had to be a rather cynical view of sex as a form of exploitation and a commodity. She likely saw it was 'either you're screwing someone, or getting screwed' in both the literal and figurative senses, which explains a great deal of her attitudes towards Perrin. Once she comes to accept how Perrin and Faile see their relationship, her past interactions with Rand probably make a lot more sense to her as well. Remember, she tried to seduce him and was firmly rejected, but at the same time, he offered her freely what she sought to gain by seduction. Meanwhile, he gives the appearance of having a more advantageous partner in mind (if Aviendha was right about how widely known his thing with Elayne was, Berelain almost certainly knew about it, which would also explain her switching targets to Perrin, and even entertaining for a moment the idea of Mat - she saw Rand as out of reach, but was not frightened away because she was still eager to work with him, so Elayne is the best explanation of her reticence), and Elayne appears to have succeeded where Berelain failed, when he tells her he intends to give her the Sun Throne (which he claimed to have selected an occupant for back when he sent the Tairens north). Faile's explanation of how Perrin (and by extension, possibly Rand) see relationships might have thus been the key to understanding a great deal that had passed before her eyes to this point, and opened her up to a whole new way of dealing with people. And right at this juncture, along comes a man who makes every vaguely heterosexual woman except his step-mother and half-sister stare in awe at his looks. That would be enough to at least catch her interest at a receptive moment, and from there, she is ripe for romance. Plus the fact that he is a good and decent person at heart and in his overt behavior, would be more of a clincher to someone who has only experience of men has been exploitation. As for what he sees in her, his own straightforward and sterling character probably make him a good judge (contrary to popular fiction tropes, genuine virtue does not blind you to disreputable behavior, it simply makes you more likely to be generous to the less honorable on the off-chance they might be open to redemption, and more forgiving of flaws - it does not remotely make one more gullible if he chooses not to be taken advantage of) of others' character, and despite their disapproval of her sexual mores, both Perrin & Rand found things to admire about her, as did Faile in spite of herself. The only character to scorn Berelain and constantly look upon her as a worthless slut is Egwene. Surprise, surprise.