Note to Romeo and Juliet, 3.5.206 "How shall that faith return again to earth, / Unless that husband send it me from heaven /
By leaving earth?"

Juliet asks how her marriage vow can return from heaven to earth [to be given to Paris], unless the husband to whom she made the vow [Romeo] goes to heaven and sends the vow back to earth.

In short: "How can I marry again unless Romeo dies first?"