That's interesting cause on my cuck playthrough Lena can't flirt with Mike and have sex with Billy. How did you achieve this?
I skipped through just now, choosing the slutty options, favoring Mike when available (instead of Robert, BJ in the club bathroom), did the game at Ivy's (use mouth). At the first asking Lena doesn't want a relationship, but still hook up. Then did 3some with Louise/Jeremy, 3some with Louise/Mike, cucked Ian, told Seymour to shove it on 2nd meeting, screwed Axel at the photo shoot, but shut him down everywhere else. Before the beach they both decide they want a relationship, at the beach Ian has no choice at this point, since he is Lena's bitch, so there is no open relationship talk, just her talking to Wade.
At the mansion I made all the flirty slutty decisions, went topless took the drugs teased Jeremy, fucked Anthony after massage, took Jeremy to the room in the afternoon, did both truth and dare. At night chose to stay outside, flirted with Mike, again with Anthony during photo shoot. Outside again talk to Jess, stay outside, talk with Zarina, tease Mike and had half sex in the pool, then get the option "have them both".
I can't get this on anything but the cuck path. To start a relationship early she cannot mess around with Jeremy, which as ffive pointed out is required for the Lena/Mike/Jeremy threesome. If it goes to relationship late at the beach, if Lena is a bad girl Ian can choose to open the relationship, sex with either is no problem. If this isn't chronalogically correct, or I missed a few steps, sorry there are a lot of choices, lol.
edit - forgot to add, had sex with Billy and Ivy too, and Jeremy crashes that party as well.