It's really interesting to think how the addition of various factors plays such incredible roles in outcomes. When almost all women would demand the same from a man (promise of marriage in case of pregnancy) before consenting to sexual activity, they had the upper hand on an individual level. But once birth control came into play and that demand was no longer necessary, they actually lost their upper hand - now men could know that they could sleep with anyone without having to worry about a baby or marriage, lowering the standards of commitment in a relationship and compelling women to have sex to keep relationships. And once the standards were lowered and birth control became a woman's choice, then any resulting children were her choice as well - essentially absolving men of responsibility.
For the specifics, read below...
The Food and Drug Administration approved the pill for contraceptive use in 1960. Over the next half-century, the marriage rate declined and the illegitimacy rate skyrocketed, Charles Murray notes in a recent Wall Street Journal essay adapted from his new book: (emphasis added)
In 1960, extremely high proportions of whites in both Belmont [Murray's metaphor for the upper middle class] and Fishtown [the working class] were married—94% in Belmont and 84% in Fishtown. In the 1970s, those percentages declined about equally in both places. Then came the great divergence. In Belmont, marriage stabilized during the mid-1980s, standing at 83% in 2010. In Fishtown, however, marriage continued to slide; as of 2010, a minority (just 48%) were married. The gap in marriage between Belmont and Fishtown grew to 35 percentage points, from just 10. . . .
In 1960, just 2% of all white births were nonmarital. When we first started recording the education level of mothers in 1970, 6% of births to white women with no more than a high-school education—women, that is, with a Fishtown education--were out of wedlock. By 2008, 44% were nonmarital. Among the college-educated women of Belmont, less than 6% of all births were out of wedlock as of 2008, up from 1% in 1970.Taranto links this up with a 1996 study:
Before 1970, the stigma of unwed motherhood was so great that few women were willing to bear children outside of marriage. The only circumstance that would cause women to engage in sexual activity was a promise of marriage in the event of pregnancy. Men were willing to make (and keep) that promise for they knew that in leaving one woman they would be unlikely to find another who would not make the same demand. Even women who would be willing to bear children out-of-wedlock could demand a promise of marriage in the event of pregnancy.
The increased availability of contraception and abortion made shotgun weddings a thing of the past. Women who were willing to get an abortion or who reliably used contraception no longer found it necessary to condition sexual relations on a promise of marriage in the event of pregnancy. But women who wanted children, who did not want an abortion for moral or religious reasons, or who were unreliable in their use of contraception found themselves pressured to participate in premarital sexual relations without being able to exact a promise of marriage in case of pregnancy. These women feared, correctly, that if they refused sexual relations, they would risk losing their partners. Sexual activity without commitment was increasingly expected in premarital relationships.
Advances in reproductive technology eroded the custom of shotgun marriage in another way. Before the sexual revolution, women had less freedom, but men were expected to assume responsibility for their welfare. Today women are more free to choose, but men have afforded themselves the comparable option. "If she is not willing to have an abortion or use contraception," the man can reason, "why should I sacrifice myself to get married?" By making the birth of the child the physical choice of the mother, the sexual revolution has made marriage and child support a social choice of the father.