So why do it? And if there is just a "friendship", shouldnt your girlfriend know?
Because we were friends beforehand and our breakups were amicable so we're friends now. It's no different than any other female friend I have, except that we dated for 6 months, half a lifetime ago.
If I thought my girlfriend wouldn't lose her mind and re-evaluate every aspect of our relationship in terms of my extremely rare interaction with someone that I went to college with, I would absolutely tell her. But, she's a woman so

.
Actually, now that I'm thinking about it, I did tell her about one of the meetups and she seemed cool with it, except for when she asked, "How did she look?" and I said, "She still looks good." That, turned into a fight.
