Let me please correct you and add something.
1. Yes, we
can. Every single character. In one playthrough. All in one (with losing some content and RPs, but it's not a deal breaker though). At the moment, DreamNow gives us such an opportunity to cheat and not get caught red handed. And there's no doubt about it, there will a choice or variety of choices for us to make in the future. So, we still have time to think and decide for ourselves how many and which girls we'd like to see around until the story comes to an end.
2. Not sometimes. This is only the case in Sara and Ashley's story. This is initially so, but then we can decide whether to make a choice of being on friends route with Sara and keep developing a relationship with Ashley, or decline Ashley's offer to become her sex mentor and choose Sara's love route.
3. Unless of course, you look forward to future threesome (triad) side storylines with certain characters (Carol + Yuki, Roxy + Nikki, Sara + Ashley (until you make the very important choices for each of both stories, as it was said in para. 2)), rejecting any character doesn't affect another (others), at the moment. You can just miss some scenes and dialogues, but by and large, these are not great losses.
So, we can choose freely to develop relationships with whom we want according to our own tastes. It will be whether a fuck-them-all route, or separate solo paths (separate threesome parhs). Your MC can either be a friendly guy, or a total jerk. It depends. DreamNow gave you а palette and colors, so paint your story