I'm with you in your general thoughts but here are some thoughts of mine. not an excuse just as an explanation: Lena had some alcohol and while she is not really drunk we know how a few drinks can lower the boundaries what a person is willing to do. One solution would be if Lena would think about what happened on the next morning and says something like "it was nice to have quick, casual sex without any strings attached but I also have this bad taste in my mouth because I made a boyfriend cheat"If Mike didn't mention his girlfriend, most of my problems with Mike's route would disappear in an instant.
that would be a good way to explain why Lena did it while at the same time show that she still has her moral standards.