I wrote a girl!Jeff story for a prompt, but it took me a while to get to the story I did write, and this is one of the handful of other ideas I thought of.
Other story I'm not writing: Mike/Jeff/girl!Jeff, possibly in an idea stolen from Time Traveler's Wife sort of way. Girl!Jeff lies down for a nap and wakes up thinking that Mike's playing a prank on her because the room is not quite right and the clothes in it are all men's clothes, and then she goes downstairs and Mike stares at her and says, "Who are you?"
She gets mad at him, and while they're being confused at each other, Jeff comes home from the grocery store. Girl!Jeff thinks this is part of the prank at first, until she figures out that not only are they not joking, but Mike's also younger than he's supposed to be. So once they figure out what's happening, she frowns and asks, "Does that mean you're gay in this universe or whatever?" Neither one of them know what she's talking about even though she's like, "If you're me, and you're Mike, then you're together, right?"
She and her Mike have been trying to get pregnant, and the doctors say there's no reason it shouldn't work, but it's just not happening. So when the three of them have sex, she tells Jeff he has to wear a condom, but fucks Mike bare.
She's there for two days, and she gets back to her own version of the house to find Mike has been increasingly worried, but mostly thinking she was just mad at him and not answering her phone because of that. She tells him about fucking the other him and the other Jeff, and he listens to her, and then says, "Do you think that means there's a universe where we're both women? I could get into that."
Jeff laughs, but they're okay, and when she misses her period, they both know there's a reason she disappeared for those two days.
- Plot Bunny of the Day: Mike/Jeff/girl!Jeff