I agree that abortion, per se, is not in the federal purview, but settling full faith and credit issues between the states is an enumerated power of Congress.
What happens when the states conflict with each other over their abortion laws, and an interstate issue arises? What happens when, for example, Texas tries to prevent a woman from crossing state lines to get an abortion? Will Texas be violating a woman's right to freedom of assembly? If a woman has an abortion in New Mexico and returns to her home in Texas, will Texas try to prosecute her? Does she have a "full faith and credit" defense arising from the laws in New Mexico?
What happens when Thomas' veiled threat to go after same-sex marriage happens? Will some states refuse to accept marriages allowed in other states? That's ripe for a "full faith and credit" crisis right there.
-PJ
Well, now I know you meant to use "patchwork" as a pejorative, my mistake.
Full faith and credit for abortion means a citizen of one state cannot be prosecuted by that state for getting an abortion in a state that permits it. Home “marriage” is harder, and will likely require separation of religious marriage from government unions. The term “marriage” belonged to the churches long before 1776.