You actually said the whole point... Who's the crazy person who would trade their girlfriend for Sienna? If it were really possible, I wouldn't give my girlfriend to the other guy. This isn't a game...
Perhaps this works as an option for horny idiots that tanks their relationship.
Layla surely isn't at the point that she's so bored that she wants to do this just to spice things up, Layla-MC only just began(Elena not even that).
Layla will no doubt appreciate if after the proposal and you go, "thanks but i only need Layla and i don't want to ever see her with another guy".
If you accept and then put effort into convincing Layla, maybe you get the scene but a bad relationship going forward.
Iris would maybe consider it
I don't think Nyx will do random combos, if there's any "group activities" it will be within the friend group, they won't exchange with a person they never met/barely know.
Iris being openly bisexual means she's a good candidate for a FFM threesome, but like i said has to be someone she knows, Alexis or Lakeyin are possible.
For example after the club if Iris-Alexis-MC went to their dorm together who knows what could've happened?
With all those "who is the better kisser?" games already happening...
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Why do you think so? Just because she likes sex? And then she would fuck a stranger because MC asked her to? No, but if you really want to give someone in return, give her Tori... she would be 100% into it...
Tori might refuse for a different reason, thinking Isaiah is too much of a nobody(the Tori path requires that you win the race afterall).
Also i don't know if Sienna will want action with just any girl from school Tori? Jasmine(Tori's BFF)? Val or Dani? Possible but we don't know...
Layla however does feel like she gets her motor running.