I know this is a common complaint about the game on the fact everyone does there own thing and also seems to be sluts but I honestly kind of enjoy it because when you apply just a little bit of logic to it makes sense. everyone in the game has there own sense of morality, for example if you look around the orcs it makes how ruksana acts and the fact she seems to be among the most 'not-loyal' of the group makes sense, she grew up and is from a culture that views sex as something as normal as a handshake (sex in public, sex of defeated opponents, sex with relatives, sex with animals as part of rituals, etc) so of coarse she would be way more open to exploring things sexually then say a human who tends to treat sexual matters as a sacred and private thing. as for meredith she makes sense to, she acts, and for the most part stays acting fairly tight when it comes to sex as you would expect from a human since again, sex tends to be treated as sacred, but the more you corrupt her the more she realizes she is able to finally let loose, she grew up in a fairly small (with the way the place seems set up) village with not to much contact with the outside world so thus didn't get much chance to explore her sexuality without risk of facing judgement, so thus when she realizes she can explore without that issue now she would turn from someone who is fairly reserved about sex to someone who is actually pretty open with it and fairly exploring.
most of the characters you meet come from either cultures that have different values from humans thus we might see it as weird or different (ruksana and red skinned girl (hate i forgot her name)) or lived in some sort of environment where they had to keep there sexuality suppressed (meridith and clawyn for example)thus when they are allowed to have sexual freedom they might go a bit crazy with it. honestly I would have to say the people who change the least and act the most to there 'true self' is ruksana (unless you corrupt her she doesn't change much, sex is fairly normal for orcs as stated), the succubus (because she was never holding back to begin with.), and the older fox/wolf sister (I am honestly sorry i forgot her name, it has been so long) and that is mainly because of the fact she tended to not be suppressed more so as she just was lacking self-confidence. (her blind sister on the other hand was keeping herself suppressed since she thought she was a burden and was constantly on the move with her sister hence why she goes fairly extreme sexually once she starts to open up, she really wanted to explore)
to be fair though that is just my interpretation of taking this, ones culture explanation could be anothers 'just a excuse to act like a slut'
Edit:just wanted to add a little point people seem to miss when throwing the above complaint, based on the way the world seems to be set up it seems the world by default has sex as treated as a more open matter and it isn't as big of a problem to be comfortable with your sexuality (heck becoming a paladin, a warrior with literal holy abilities requires interacting sexually with a paladin, if that doesn't say the very gods have a fairly lax view on sexuality i don't know what does.) thus it would probally not be that much of a surprise a lot of the characters turn out to eventually become a lot more open about sex.