[Ant.]

"Put it in there" is too long. The only good thing about that scene was the music

"Open it" is snappy. It rolls off the tongue, when you're gasping for breath during intercourse, it only makes sense that you'd plead him to open it. It suggests a real sense of intimacy - are you really going for it if you have enough breath to utter a sentence?
Also, "open it" is better as "it" can only mean Kim's vagina, or maybe her butt if Steve likes anal - she's not Simone, she's not gonna open a packet of crisp in bed.

"Put it in there" - put WHAT in there? We don't know this Jake, we have no emotional attachment to him. The statement is as empty as Ellie's vagina - maybe he has a foot fetish and the "it" are his toes. Maybe he put his fingers in there. Or his fist. Maybe he went classic BB style and shoved a bottle up her pussy. There's too much ambiguity and for what? There's no reward with it.

Besides, "open it" just sounds better. "Ellie's gonna #openit" > "Sam's gonna #putitinthere"

Also nothing will beat Kimberley waiting for Steven behind the eye as Emma says "open it". How would you replicate that with "put it in there"?