Patriarchy causes unplanned pregnancies by preventing women from having the knowledge and power to make the sexual choices that are right for them. It emphasises penis in vagina sex as the only kind of 'real' sex even though it is riskier for women than many other equally or more enjoyable sexual acts. It puts women in a situations when they have unwanted sex due to the unequal power dynamic between men and women. It infantilises women rather than encouraging them to take responsibility for their reproduction. Men are absolved of the responsibility to control their fertility because pregnancy is seen as a woman's problem and hers alone.
Even if you thought that abortion were completely morally neutral surely millions of women paying hundreds of dollars each to have a not particularly pleasant medical procedure should tell you that there's something seriously wrong with the position of women in the US.
Now, I acknowledge that even if all women were empowered to control their reproductive abilities there would still be unplanned pregnancies but there would be less. Patriarchy also means that women who do get pregnant when they didn't mean to are more likely to feel that the consequences of not having an abortion are not bearable. Our major institutions are designed for men rather than women. Despite the advances that feminism has made we are still at the stage where women are adapting themselves to fit into male institutions rather than adapting the institutions to fit them. As one of the defining differences between men and women is that men can't get pregnant, our institutions are designed for people who don't get pregnant. So women are frequently forced to drop out of education and employment because these institution will not adapt to the fact that some people get pregnant. What kind of a 'choice' is it if the alternative to abortion is exclusion from all of the institutions of political, economic and social power?
Then there is the way society expects women to be sexually available and then demonises them if that sexual activity results in pregnancy and the way that we are so intolerant of people not fitting into our ideas of perfection that the majority of fetuses that are found to have Downs Syndrome are killed before they are born. Let's not even get into the way that it is very easy for men to simply not contribute anything to the financial support of the children they father and it is very difficult for the mothers of their children to take action to force them to pay their share.
Abortion doesn't solve any of these problems; it simply makes them less visible. It pushes the burden of 'dealing' with them onto women who are then expected to be thankful that had the 'choice' to have an abortion rather than be forced into destitution by a system that only accommodates women if they don't dare to stop being doormats or pretending to be men.
That's why I get so annoyed when I see pro choice feminist schmucks kidding themselves that they've achieved some kind of feminist utopia by being allowed to use their money, their bodies and their offspring to cover up the huge injustices of our society.