This has actually been studied, in the formal research followed by paper published in academic journal way. (I’m bad at googling, so no link.)
Most of the reasons were as mentioned/suspected: longer time needed due to anatomy/dress styles, more time spent on grooming, some chit chat over ‘do you like the guy?’ and planning how/if to split up into pairs vs. a foursome, etc.
But there was one that was both totally unexpected and yet so true it made me wince in recollection:
Sometimes the women linger in the bathroom because it gives them a chance to talk about what THEY want to talk about.
Think about it.
Many girls are brought up to be ‘good conversationalists’, which means mostly smiling and saying ‘how interesting’ and ‘what a clever idea’ and most of all, making sure you talk about the other person and what he/she finds interesting.
Usually this works well, but sometimes if you put two girls and two men at a table, the men sort of compete with each other to be the ‘star’ of the conversation, the one holding the most attention. This can either get nasty and unpleasant for the onlookers, or the conversation can get cemented into some subject that both guys happen to like, be it some sport, politics, kungfu movies, a type of music…
And after that has gone on long enough for the women to feel they can’t take it any longer, one of them suggests hitting the rest room. Where before, while and after the tinkling and grooming, they can swap diet tips or recipes or commiserate over their cramps or gripe about their bosses or talk about a DIFFERENT sport/type of music/horror movies/whatever. And then they can take deep breaths and go out and chirp, “So, did you decide which band has the better drummer?”
F/F conversation just has a more give and take ethos, you both try to be sure the other girl gets her chance to talk. Way different from most M/F conversation, at least until that M/F pairing has become old friends.
Mind: I’m not saying this is all the guy’s fault. If his partner is saying “how fascinating” as he talks about building his cabin in the woods, it’s not unreasonable for him to think he should keep talking about it. He’s ‘doing his part’, he’s ‘keeping the conversation going’, he’s doing his best to ‘entertain her.’ Isn’t that what he’s been told he should do?
It’s just an unhappy outcome that can happen when two people have internalized different ‘rules’ for how a conversation should go.