After thinking a bit about it, I guess the author wanted to show a reversal in the way the Manager's relationship played until now.
Previously, she picked up some guy, had sex, and left in the morning before him, no strings attached. It works for some men, so why not for some women?
Here, she got carried away because the atmosphere and feels were better than she expected and she found herself in the position of her previous partners, left hanging.
But all is well that ends well, because this one partner comes back for her, foreshadowing a more fulfilling relationship.
So, after thinking about it, even though it's not something I find appealing, her picking up her sex partners in parks isn't worse than doing it in bars, clubs or any social gathering.