I don't really remember much about what happens after that because I did Lars' route a while ago, but from what I can remember Lars is back to his old self, so I'd say that when they finish Lars' route completely there won't be any polyamory or whatever it's called, at least from what I've seen and I think Lars just did it out of nowhere and I don't think it'll become more... SEX. (That's why my favorite is Marc)