The key word is finishing!
When a story just keeps going and going and going, you have little choice but to keep expanding its scope. You can't have your protagonist keep fucking the same 3 people over and over, there has to be progress. Of course, there are multiple axes of progression. You can go from flirting to kissing to heavy petting to full-blown sex, but at some point you've done it all, and further escalation becomes... well, let's just say unappealing. At that point you'll probably have to resort to multiple partners. First the protagonist seduces her childhood sweetheart, then the buxom neighbour, then her brother or mother (probably those first, actually, and probably both), and before long you get the scenario you're sketching, where the story just grows out of control (and sometimes other things do too). Branching only encourages this kind of uncontrolled narrative sprawl.
But when you keep in mind your story has to end, that puts an upper limit on how broad you can reasonably make it. Unless you're planning a 10-volume fantasy epic, you can't hope to tie together dozens of disparate storylines and characters into a cohesive, satisfying ending. This will also force you to focus on story (your arc has a beginning, a middle, and an end) and a few central relationships (say from strangers to lovers to soulmates), rather than just piling more and more meat onto your orgy.
I strongly believe that you can't truly call yourself a writer until you've written an ending.