The Arkansas voters banned unmarried couples from becoming foster parents. Although it was aimed at same sex couples, my opinion is that the impact on opposite sex unmarried couples is more significant. We already know that children do better with married couples than with cohabiting couples. The state should absolutely take this fact into account in choosing foster parents. "Opponents argued it would make it harder for the state to find the foster parents it needs to take care of children." I don't consider that a very strong argument. The states should recruit more married couples and make foster care an appealing proposition for them, rather than lowering their standards.
As a personal note, I have seen some unmarried cohabiting opposite sex couples do adoptions and foster care. It doesn't work very well. The problem is attachment. Many, many adopted and foster children have difficulty attaching to loving adults. Their ability to attach has been compromised by the trauma they've already been through. They need the most stable possible people to connect with. The unmarried couple implicitly says, "I do not entrust myself to my partner. My partner does not entrust himself to you. But we want you, the wounded child, to entrust yourself to me and to him." States should not be using cohabiting couples.