I've been with this girl "officially" for about a month.

She's mid twenties. Good looking. Good job. Really good family relationship with her father and mother. She takes care of me, buys me things, makes me stuff, etc etc. Pretty much best LTR I've had.

When we started dating she said that one of her coworkers (we'll call him Mike) started being super distant with her. Not talking to her as much etc. Before we started dating they were really close friends. Traveled together. She met his girlfriend who lives in another city about 4 hours away. Etc etc.

I had a gut feeling that something happened between the two of them but she stated that it's always been really platonic and just friendship. She was upset that Mike let her new LTR affect their friendship. I thought it was pretty scummy of him to be all distant and also super shady since he has a girlfriend and all. Although I did prefer it this way because I truly feel that guys can't be "just friends" with attractive girls.

Anyway, super long story short I found out this morning that there was more to that. Apparently a month before her and I started dating they got really drunk and slept together. She said it was just once and that they agreed to act like it never happened.

Of course I'm fucked pissed because she lied to me and on top of that, she's been actively trying to get their friendship back together and get things "back to normal" ever since he started acting distant. Recently though he's been acting a more normal and she even asked him where we should get a hotel when we go on holiday.

She's the one that fucked up. His reaction to keeping distance was correct as he probably felt guilty and awkward for what he and her did. His girlfriend doesn't know he cheated.

I mean I understand this happened before we started dating so should I let it go? The fact that she met Mike's girlfriend and knew they were in a relationship and still fucked him is really messed up. Also messed up that she continued to try to be friends with him while her and I have been dating.

I called her out on all of this and she's done almost no hamstering except blaming it on alcohol. She agreed to not drink anymore. She's blocked him on Facebook. She's also said she won't talk to him at work anymore except for business and will actively look for another job.

She's been crying all day and trying to talk it out and been apologizing non stop. I feel like she's pretty genuine but I'm obviously really upset about being lied to and also upset about her apparently shitty morals.

We've been together for a little over a month. Her friends and family love me. My friends love her. She's been 100% great so far minus this one issue. Just feeling super conflicted.