Yabu - humans can refer to their own social events however the fuck they like - whether it's role play weekends, book club meetings, night out with the girls, night out with friends, boys night, date night, family day out, family holiday, engagement party or even the dreaded baby shower, it is just words people use to describe their own evening in a way that conveys what the event is - if you understand what they mean then job done imo girls night out infers a night out with one or a group of female friend/s.