I contemplate divorce every day. It tugs on my sleeve each morning when my husband, Will, greets me in his chipper, smug morning-person voice, because after 16 years of waking up together, he still hasn't quite pieced out that I'm not viable before 10 a.m. ..snip.. Maybe one day, marriage -- like the human appendix, male nipples, or your pinky toes -- will become a vestigial structure that will, in a millennium or two, be obsolete. Our great-great-great-grandchildren's grandchildren will ask each other in passing, "Remember marriage? What was its function again? Was it that maladaptive organ that intermittently produced...