You can always deny dating or sex with a char before it begins, and after there are points where you can break off the relationships, change their nature sometimes (fling, romance, or by messing around and getting caught). Cheeky and co. did a great job of leaving that kinda stuff up to the player and we plan on continuing that. This is all regardless of what points you accumulate, although generally hitting on characters to generate lust points can lead into their pants (if you don't say no) and some may assume a relationship (kinda like real life, go figs lol).
As far as the single play through question. I figure your trying to have a main save you can use to go back and cut down replay time and see all the content while remaining loyal? It's not really gonna work. There is a lot of branching that happens throughout that will cause you to miss content as you go if your not seeing a girl, and some you will miss if your not seeing several of them at once. The good news is our guy Phalzyr has a mod that will help with that.
https://f95zone.to/threads/where-the-heart-is-ep-15-cheekygimp.10549/page-291#post-1764985