My two cents;
To lengthen any story you either create conflict or story point. There are probably other methods but those are the two I can readily think of right now.
A - Conflict is essentially a problem your character(s) face, address, and fix. The resolution may or may not be favorable for all parties involved. The problem unravels and develops the story. The characters affect it and are affect by it. All of this builds up for the resolution, which can also be your plot twist. Once you finish a conflict, you either end the story or simply create another conflict.
B - If your story is more of a 'slice of life' kind of story, then you may want to present your character(s) with a situation where they want to make a decision, I call this story point. This could lead to conflict, or could just lead to another point where they make another decision.
A successful story combines both elements. If you rely too much on creating conflict, your characters lose autonomy. Save for one character, the others become pieces in a chessboard without any will or power to make their own decisions. If you rely too much on creating story points without creating sufficient conflict, then your story becomes increasingly bland and repetitive, and your characters static as opposed to dynamic. It might even become unrealistic as your characters survive life without facing any obstacles. Watch any good movie and you'll see this is how stories are usually built. The only addition they make is the inclusion of a wow factor, something that is unexpected and is intended to steer the story somewhere unanticipated - a plot twist.
If your story is essentially a dating story where you have two characters dating and then bedding, then you probably don't have much to add. It is always better to flesh out your character's past, present and future. Expand on their families, life plans, circle of friends, and introduce multiple characters to the cooking pot. Instead of two people in a date, make it a love triangle, give someone an opportunity to work somewhere far, introduce pregnancy, family drama, Or just have your character run away during sex and get hit by a bus, etc..
If all else fails, maybe its time to end the story.