Checked the code. The only variable that makes Lena not care about Mike's girlfriend is wearing Ivy's sexy top... Really?... Do sexy clothes turn girls into sluts?...
In addition, Lena's internal monologue during the dance:
"Him having a girlfriend didn't mean I couldn't play a bit and have some fun.
It's not like I was doing anything wrong..."
"Besides, I had chosen to dress this way tonight for a reason. I enjoyed the attention and the flirting."
"I could blame the alcohol, and even though it's true I don't need much to get tipsy, I hadn't drank nearly enough to act out of character."
"But I guess I was just in need of feeling sexy and empowered once again..."
"I for once was surely enjoying Mike's reaction to me."
"It was a fun little game I hadn't played in a long time, and I was glad to see I hadn't gotten rusty at all."
No internal justification for actually bringing Mike home.
"He knew what he was signing up for as much as I knew what I was offering." - Ok, but WHY do you think it's a good idea, Lena? It's kind of a biggie...
Later, during sex.
Mike "You're such a naughty girl, Lena."
Lena "You're one to talk. You have a girlfriend, yet you're letting me suck your cock..."
Mike "Well, that's... I find it hard to say no to you."
"I giggled."
Lenal "Don't worry,
I'm not judging you. If anything, I'm glad you're a naughty boy..."
Okay, I think now it's clear that Lena is a hypocrite.