IIRC, if you turn a character down, that's a permanent choice. There is no "pursue her later" option. If you turn Ami down in that one opportunity, she is cut off, all her sex scenes fail, and you have a very different relationship with Ami.
sooo ive just finished going through all the missable events on the wiki. and from what i can tell there is no "bad consequences" for pursuing ami from the beginning other than a potential during a Noriko invite over scene where Ami can catch you in a lie if you dont do some prompts first? (im not there yet. this just what wiki says)
otherwise it seems pursuing any of the older women has no negatives, and might even all be positives for their linked main characters (Sara, Haruka, Maki).
so basically from what i can tell all scenes are meant to be Seen, and the only questionable one is that endgame noriko invite scene which sounds passable with ami relationship
there are many scenes you can miss but its all about failing to grind enough points in almost all cases.
just a few are based on some choices in game (dont say any Kill Kirin option in FMK game. Dont finger chika before the first beach vacation. Choose Miku during Tip Your Bartender. Win the lottery. swallow a spider. see "Abyss" in Yuki track before you get to far with chika. couple others)
so basically you are meant to pursue everyone. there are no (so far as i can tell) "you made this choice to bang person A therefore you cant bang person B anymore" unlike many games of this style.
so that was what was throwing me off. took me quite a few hours to get full understanding of this, and make a rough guide for avoiding missing any scenes throughout the story.
i guess there is a kind of "100% single playthrough" achievement then?
will be devastated if future update changes this LOL