In all or most states, children born of married parents are presumed by law to be the biological offspring of the husband, unless he chooses to challenge paternity.
The Mormon Church has issued at statement regarding the Prop 8 ruling today. They, along with Catholics and others, were instrumental in getting out the vote for prop 8.
Yes, that is true (as I think I mentioned). But a child born to an unmarried woman, fathered by a married man, does not create a parental responsibility for the wife of the father. It is physically impossible that the wife is the natural parent. Just as it is physically impossible for a spouse in a same-sex marriage to be the natural parent of children born to their partner.
So, to pass court muster, California must change its law to only create the presumption of paternity for male-female marriages, and exclude the presumption for same-sex couples. Then they have a valid reason for having a legal difference between a "domestic partnership" and a marriage.
Other states that have not given such sweeping rights to same-sex couples are unaffected by this narrowly-drawn decision.