Well, personally, there is nothing wrong with your gf/wife being slutty. As long as she's not the village bicycle. THAT is a huge nono. In the context of this game, you can have Marianne be monogamous to you (or as monogamous as you can get I suppose): skip the event in which you catch her BJing her now ex (not sure if canonically it still happens), tell her not to mention anything to poor Pete at the beginning of CH3, followed by telling her to have a clean break at the coffee shop.
I have no issues with someone being slutty, just not someone I would consider wife material. It was semi said in jest. Considering how much of a slut my MC is (going to be).
Nevertheless someone willing to cheat on their spouse and having little to no remorse about it is a giant red flag. There is no reason to believe it won’t happen again in the future and you’ll be on the cheated on side of things. That is the primary reason for me, not her being slutty.
She also constantly flashes that dude you only see once at the bar when you go there with her and Peter. Again she’s fun, especially when you get to cuck Peter. Just not someone I would ever consider for a monogamous relationship.
The first sentence proved you actually stopped reading the story sometime before chapter 3.
Marianne and Jane are basically soulmate/bff's at this point due to the shared sorrow they had with their dad's. Sure you can choose to not chase her (I think?), but why would you? And how has she been a 'slut'? All indications are she's been very faithful to her boyfriend(s), not sure if Peter was her first, and she felt bad enough she broke up with him as soon as she realized who she really loved.
She's a free spirit who's down to experiment and push the borders WITH THE MC. This is not like she's out banging the entire team, she's devoted but open minded. You WANT a partner who is devoted to you but open to try all sorts of potentially fun sexual options.
You want that at lest, if you're not sorta repressed yourself and maybe insecure as an individual.
Again nothing with being slutty, me not considering her wife material has no reflection on her being a great friend. I absolutely agree that she is. I would very much keep that relationship going. I want her to be slutty, I want to keep fucking her and I want to keep involving her BF. In essence she’s a good slut, my MC is a slut. I want them to keep being sluts. You can love someone without making them your wife.
Just don't raise her [love for others] stat. Marianne is not the only character with this potential stat. Anna for exemple has this stat too. If you don't raise this stat Marianne will be exclusive to MC. I'm on Marianne's exclusive [love] route and it's great

She broke up with her former BF peacefully.
It would be sad if she is the ONLY one that gets impregnated by MC tho
Said most of it above already. I want her to be a slut, I don’t want to be exclusive with her. I don’t understand why everyone thinks saying not wife material means I don’t want her at all. Stop trying to ruin my slutty dreams.
And yes, hopefully we get to help a lot of couples and turning all their boyfriends and husbands into cucks that have to raise MC’s children.