I guess like everything I do this'll be rambly. I've been thinking about this lately too, possibly because I'm new here and the branching is so beautifully novel to me and I
love it so much that I've gone on kind of a self-indulgent ride so far. It's kind of surprised me how insanely enriching and satisfying I find it, to be able to use so many of my ideas for characters and interactions. If it doesn't fit into one, I can jot it down and use it on another thread. So for now at least, I absolutely love the branching possibilities for the freedom of exploration they give me. However the obvious downside to that is the resulting sprawl can get a bit unmanageable. I know you mentioned you haven't delved into branching at this point, so I can for sure see why you'd hesitate calling something ended when there's only one thread. But I second everybody who said go for it if you have an idea for an ending. Just because your story hasn't branched yet doesn't mean it can't now. I think you can have your cake and eat it too in this case.
Not sure anybody cares but in the spirit of sorta "thinking aloud" I thought I'd discuss my thought process about controlling thread-spread and considering my tendency to branch and anticipating the need to control for that. In my first story I start with a single evening and then have a lot of threads splitting off with some degree of spread where most of them lead toward something intimate with her brother, this going variously better or worse depending on MC's choices, and some of the newer ones trend toward possibilities where the brotherfucker thing doesn't get off the ground but here's this other guy, blah. It's been on my mind how to get these under control and so far the approach I've been most drawn to is a technique I saw in Amanda Clover's
Suburban Sex Kitten doorstop masterpiece, where absolutely all kinds of different story threads get yoked together at time intervals, i.e. MC fucks hobos in a boxcar or gives the convenience store clerk a titty job or fucks her elderly neighbor in his backyard, but as long as she doesn't get knocked up and earn a BAD END, all threads funnel to a "Continue" point when the morning ends, and new threads branch from there. Same with the evening.
I've been loosely keeping track of certain things like "does her brother know who she lost her virginity to" or "how frankly have they talked about what their actions mean for their relationship long term" or "how many irresponsible creampies can one desperately horny teen couple get away with before get themselves in real trouble" for my various threads in the hope that I can route them roughly into several thematic buckets depending on where the characters are situated relationship-wise, who knows what, and how reckless they've been, etc.
So, thus far my plan is roughly to attempt bringing all these threads up to a certain point in chronological time (end of spring break week say) and link each of them to one of a handful of scenarios where we resume. This is a huge if really considering how bad I am at reining in, but I guess where I'm really going with this longwinded shit is that I feel your indecision keenly and offer you my moral support if none of this was actually worthwhile.
TLDR:
I see you Dansak
