I was a secondary for a while, and my situation was kind of complicated because I already knew the girlfriend of the guy who I was seeing. We didn't need to meet for the first time because she already knew me. However, it'd been about 11 months since we'd seen or talked to each other (when I started dating her boyfriend) so it was necessary to "re-meet" each other, so to speak. That took place after the guy's and my fourth date. He and I had also had sex on our fourth date. So what I'm assuming happened is that...he told her that we had sex (they probably agreed to tell each other when carnal relations took place), and that immediately compelled her to want to meet up with me. The morning after the night I had sex with him (well, around 11 a.m. that morning), I got a text from her that said "Coffee?
If I were in the position she was in, I'd probably want to meet around then, too, as soon as you know the person is gonna be someone your boyfriend or girlfriend really plans to date for a while. Although, personally, I would just as soon not meet them, also, not until later. I'm happy with a "don't ask don't tell" thing for longer than most people. I think, really, you save yourself some weird feelings going that route for a while. People look down on it a lot, as if it's somehow wrong, but I don't think it has to be wrong. Just keeps things private for a while.