This made me imagine a scenario:
Ivy and Ian( who is dating/couple with Lena) on a date behind Lena back in some restaurant and in that same restaurant Lena is on date with Holly.
Ian and Ivy playing bang bang in WC in one stall while Lena and Holly in the very next stall. Four of them hear each other's moaning, shrieks of pleasure. After finishing their business four of them came out of stall at the same time discovering this whole Quadruple.
I can see it. Similarly I can imagine Holly and Lena dating while Ian and Ivy are a thing and Holly kinda has some unrequited feelings for Ian and Lena teasingly asks if she had to the chance to sleep with Ian and her would she. Holly naturally would blush and be flustered and admit she wouldn't mind the idea. Days later Lena playfully mentions what Holly said to Ivy, as a joke of course, which cause Ivy to consider it.
Exchange-Lena, Holly, Ivy, Ian chilling at the park.
Ivy-"So Holly... Wanna bang Ian?"
Holly-"Eh?! I-I mean I couldn't do that to you! I'm happily with Lena!"
Ivy-"Don't deny it girl. I know Lena has been interested in Ian for quite a while. Haven't you Lena?"
Lena, slightly blushing-"I won't deny I'm not curious..."
Ivy, looking over at Ian-"Hey loverboy. You down to fucking my two best girlfriends?"
Holly, turning bright red-"D-d-do we hav-"
Ivy-"Shush you! Let Mommy Ivy work here."
Ian-"Don't know... You in this aswell Sugar Tits?"
Lena, Holly-"Sugar Tits?!"
Ivy, cheshire grinning-"Naturally."