Seeing as reddit is just usenet of the 80s and early 90s on a web page in 2014, both in application, discussion topics, and more or less in culture, one lesson from usenet readers of the dark ages was there are or should be no default groups.
I don't remember any default usenet groups in nn, tin, gnus... The same social pressures that lead to no defaults then, most likely apply now, even if the tech is slightly different.
"a sub that's supposed to be by and for women."
I lurked there and read a lot. Interesting to see their opinions vs mine. Eventually it got boring. Encouraging half or more of the new user population to lurk before posting was another usenet thing. So looking at it that way, it might be a good idea to force noobs to read, at least till they learn how to remove a default subreddit.
So I've presented two arguments, one for and one against. I think the argument against is slightly stronger although I could appreciate a good alternative interpretation.
The news protocol specifically included a recommendation mechanism for "default subscribed groups", so that a news administrator could point new users to some important groups.
In Gnus that behaviour is triggered by the variable gnus-default-subscribed-newsgroups.
I'm certain that nn, tin and whatnot supported it, as well.
This is one of the big weaknesses of Reddit right now. As far as I can tell, who gets to moderate powerful subreddits comes down the the accident of history of who happened to think of it first. Several of the subreddits I frequent have had moderators slam down rules that most of the community disagree with (just for example /r/Android has a rule that you cannot post "questions", it is only for "news about Android" ...). This leads to a high variability in the quality of the moderation and curation of various subreddits. I'm not sure what the best alternative idea woudl be, but something better than "whoever thought of it first" should be possible.
With Reddit, you at least have a chance of having a semi-decent group of moderators who can care for and foster their community without it turning into an anal retentive hell.
Once you couple that with the general population skewing fairly male, young, not necessarily good with social norms, and a strong pseudo-libertarian streak, it just doesn't seem like a good combination with a sub that's supposed to be by and for women.
I'm curious why the mods decided to let it become a default.