Not long ago, same-sex couples had to jump through endless hoops to make their relationships even close to legal. Happily, those days are over. Same-sex couples no longer have to operate as outlaws--they too can have in-laws But here's the rub: many gay and lesbian couples, accustomed to living off-grid, are so thrilled to have the benefits of marriage that they gleefully jump into marriage without fully understanding the consequences.

In her first book, Before I Do, leading gay rights attorney Elizabeth F. Schwartz spells out the range of practical considerations couples should...