Who is Karen's role model? - surely her mother? Who must have married and had children really young. Then perhaps it wasn't the studies at Oxford but the academics - meeting women still more likely to be unmarried and totally dedicated to the scholarly life - she wouldn't be the only woman to think 'that's not what I want and is that what I've let myself in for?
She's had numerous teacher role models at school, though -- Latimer and Miss Cromwell, not to mention Miss Keith, are all, one imagines, graduates, quite possibly Oxbridge, and are all presented as donnish. It's at least as easy to see them as Karen's role models as the rather cipher-ish mother she sees only in the vacations, and only seems to emerge as a personality when she blows the proceeds of a family tiara on buying Ginty Catkin and herself Chocbar, and appearing at the hunt sidesaddle in a riding habit. 
And while 1967 was still five years or so away from Brasenose, Jesus, Wadham and whichever other ones it was starting to admit women, women had been full members of the university since 1920, and the first women's colleges had existed since the late 19thc.
Absolutely it was still unusual, but I think it would be anachronistic to necessarily imagine Karen was having the same kind of thoughts about the 'unnaturalness' of a college of unmarried women as Dorothy L Sayers gives her characters at Shrewsbury in the 1930s. Or that even the most loved-up teenager could have thought that being a housewife and looking after three grief-stricken children you've never really met was that attractive a prospect?
Only, of course, she does, so we look around for reasons.
Perhaps Edwin was wondrous in the sack. 