There are a couple of reasons McCaskill won. The stem cell amendment pulled out some extra lib voters, including, as you astutely point out, single women. But Southwest MO and rural MO did not go as strongly for Talent as a statewide (R) needs to overcome the urban centers.
Talent is a decent, true conservative guy, but a bumbling, inept candidate. He did not distinguish himself in debates, often looking childish. After his loss to Pee-Wee Holden in 2000, he should not have been pushed for the Senate.
McCaskill won by default — a warning to the Republicans nationwide.
McCaskill took about 5% more of the vote in southwest Missouri, which was the difference. A lot of those were first time women voters and swing voters on stem cell research.