Because that's who she is, Meredith likes flowers, books and having sex with extra planar being. And animals. And cabbage. You as a player has no power to change it, that's a power of the developers alone and they decided to make her a slut. If you want a faithful waifu try the dog, you can tell her to stop sucking mino dick and she will obey.
Clawyn doesn't fuck around much either. If I remember it's a couple of threesome scenes and a scene with some squirrels if you neglect her.
Rest isn't aimed at you Red, more the people complaining.
Meredith has lived quite a sheltered life and discovers a joy of sex after fucking the MC. You can calm her down if she goes to corrupt but she still wants to experiment and live a little. Ruk's an Ork, they like to fuck as does Tish it's just part of their nature. Succubus goes without saying she's going to fuck around, she needs sex to live. Cerys has finally found people comfortable around her, she isn't going to say no after years of being pent up. You can stop Saren but she's a virgin that wants to explore her sexuality.
None of them do it to NTR the MC, they don't expect anything relationship wise until you propose at the end. Until then everyone, including the MC who fucks around more than anyone, is going to have a healthy sex life.
I can understand shitty arguments like this in a pure romance game but this is an over the top RPG with everyone fucking everything. Some people are far too sensitive and really shouldn't be playing porn games, they obviously aren't mature enough to handle the content.