Sobering numbers on out-of-wedlock births. I
Also, is it just me, or is there something a bit laughable about the fact that a majority of Americans say that it's OK to have children out of wedlock, yet very few who will claim that having an affair is acceptable? I'm not saying that it's incoherent, I just think that it's telling that more of us are bothered by a betrayal of our romantic ideals about marriage than we are about the failure of marriage as a structure for sustaining stable families.
I think that's usually been the case, historically and cross-culturally, that adultery is judged more harshly than having a child outside of marriage. Marriage typically involves an (often explicit) promise to faithfulness, which also works to safeguard the marriage as stable for any children involved. It can also be a matter of wanting to show some respect to the parent-child relationship in whatever form it takes, and/or not wanting to judge those who are doing their best to raise a child as a single parent - especially as in the present day when most people know someone in that situation. Plus there are many who do not want to encourage abortion by making it unacceptable to have a child outside of marriage.