<human>: I've been dating this girl for about a year now fairly seriously -- we live together and get along great. She told me recently that she thinks she wants to marry me. Just based on how the relationship is currently progressing, everything is great.

**Here's the problem:** something that I shouldn't know about but do. When we first started dating the first time around (we broke up then got back together), she went to go "hang out" with a guy friend whose girlfriend was out of town (note: that girlfriend is now his wife). What actually happened: he booked a hotel room and she spent the night with him. (And, of course, they had sex.) She's very clearly keeping this a secret from me, for obvious reasons, but it bothers me a lot. Not just because it happened while we were still dating the first time, but because I hate the idea of her willingly and excitedly going along with a guy cheating on his girlfriend. 

Everything is fine at this point, but I'm finding that I'm still a little distrustful at times, like when she says she's "just going to go see an old friend for a while." It's getting better, and I'm more trusting now, but that thing, which she and the guy are keeping entirely secret between them, still bothers me. On one level, I want to confront her and yell at her about it, because it's an awful thing to do. But on the other hand, it's in the past, I know intellectually that I shouldn't (I think) worry about her cheating on me now, and I'm happy with the relationship as it is. What do I do?

Give me the gist of the text in a nutshell
<bot>: Giving a relationship a second shot and it's going well, but I hate the idea of her helping a guy cheat on his gf.