Sexism was rampant in Elizabethan society. Women were categorized by extremes, as either virgins or wanton women (promiscuous)--no middle ground. Women who talked a lot were placed in the wanton category: that's why there are so many interchangeable references to "tongue" and "tails" in Shakespeare's "battle of the sexes" comedies (Much Ado, Taming of the Shrew).
Also, if a woman were not married young--and I mean young (ages 13-16)--she began to garner other titles: "maid," "cold," and "fussy." I've seen Beatrice played as old as Hero's mother. So, Beatrice has two strikes against her: she speaks a lot (against men and marriage) and she is older than the ideal woman (Hero).
Look at what Beatrice says when speaking to Benedick: "I had rather hear my dog bark at a crow than a man swear he loves me." She's very defensive and witty, guarding herself against attack by disparaging the romantic, saccharin version of love.
Look what Benedick says in response:"I would my horse had the speed of your tongue, and so good a continuer." Though he doesn't call her "wanton," the analogy is clear: she who speaks so much is promiscuous with tongue and tail.
Look what Beatrice says to the Prince: "Good Lord, for alliance! Thus goes every one to the world but I, and I am sunburnt; I may sit in a corner and cry heigh-ho for a husband!" Notice, she acknowledges her status as "sunburnt" and "in a corner." She knows she is an outsider and disenfranchised.
Soon after, the men jest: "She cannot endure to hear tell of a husband." And, "O, by no means: she mocks all her wooers out of suit." And then, "O Lord, my lord, if they were but a week married, they would talk themselves mad." It is clear: he who marries Beatrice will go crazy with all her nagging.
So, by today's standards, Beatrice is a modern woman: she is not afraid to speak or to defend herself and Hero against men, and she waits until she is middle-age to marry. Shakespeare makes light of a serious predicament for women in order to change society's view of them--that's the point of comedy. He recognized the double standards and impossible expectations placed on older, unmarried talkative women.