Subject says it all, really. Been dating a cute new coworker for about 3 weeks. She's got this male friend she hangs out with a lot, but I'm trying to be cool and not the jealous type and got to know him and he seems decent enough. This weekend we were all at a party together and I wound up asking him if they'd ever slept together and he said yes, but they stopped when I started dating her. He said they had agreed to those terms when they started. Something about how he's almost 30 and she's 19 and they're in different places in their lives so a relationship wouldn't have worked, but they were still friends. But also he said he doesn't have anyone else currently, but then he made what I think was a joke about how he was working on an angle with some other chick at the party that he pointed out. He said he was happy we were together but I still feel uneasy about the fact that he's hanging out with my girlfriend and that they've fucked. I was also kind of concerned that she didn't tell me that herself, but I keep going back and forth about whether to ask her about it or not.

Am I being unreasonable here? Should I do something about this or should I get over it?