What is your opinion on having platonic friendships with the opposite when you?

are in a manogamous relationship, and more specifically do you think my girlfriend of 4 years is being ridiculous: she says because i made this a strict rule for her since the beginning of the relationship (something i just don't won't to deal with/don't approve of) that i am having a double standard just because i have old friends (people i was already friends with before her that i stay in touch with, such as women that were friends with both my former wife and myself (my wife is dead), and were not one on one relationships, and women that i've been friends with since my teens/ keep their phone numbers, talk to occasionally/embrace when i run into them, etc. (i'm 40-ish, my girfriend is a little younger and i think immature in this case, i'm a grown up i shouldn't have to explain myself)