Re: Am I being reasonable?
OK I have a bit of a different perspective. I've been through a divorce, and it took me a LONG time to tell my coworkers about it, even though I had already started dating someone else as I was going through the divorce. There can be quite a bit of shame and embarassment involved, and just a few months isn't really long enough to overcome that. It might be easier for some, and maybe I'm just a bit immature (or I don't like to be a "quitter" so to speak), but it took me about 9 months before I really started talking about it. And it took about that long before my family knew I was seeing someone else. Was she still married and living with her husband when you two started dating?
Another question -- does this other guy -- the coworker who is hitting on her--know that she is getting (or is already - which is it) divorced? Or does he think she's still married? It seems to me, based on your timeline, that he asked to have sex with her while she was still married (you say it was a year ago) and he knew she was married. If that's the case, then I don't think it would matter at all to him that you're now her boyfriend.
And if that's the case and if I were you, I'd of gone to that party. I mean, you should be able to trust her and all that, but still, just to be sure. And, especially if these people are your friends, too.
What would happen if you spilled the beans to these people? Would she dump you? You probably want to talk to her about this soon. While it may be that she was embarassed or felt it might be preceived as "too soon" for her to start dating someone else, if it has now been many months, it is time to be open about it all.