So I've been thinking off and on about the comic where imaginary!Frankie was a thing, I can dump it if anyone wants.
She was created by a boy who's maybe 7 or 8 years old, and she was adopted like immediately by a girl with three older brothers, but it wouldn't be hard to imagine her being adopted by a boy at some point, or the girls' brothers "borrowing" her for obvious reasons.
So, what would having sex with her be like? Probably... odd. Like, she was created by an 8 year-old, how many kids that age have any real understanding of sex or sexual attraction? Not many, and the ones who do I'm pretty sure wouldn't come up with an imaginary friend like imaginary!Frankie.
Thing is, I don't spend much time around kids, so I'm not sure what sorts of things an 8 year-old might think "sex" is (I don't think I even knew the word when I was 8, except as a way to differentiate boys from girls.) Probably i!Frankie would consider it to be stuff like hugging and kissing, maybe specifically in a bed. I doubt she'd have any definition (i.e. no nipples or vagina.) She'd basically be a life-size Barbie doll.