SAN FRANCISCO  It was not the type of question that one usually hears from lawyers in federal court.

“What does it mean,” Theodore B. Olson asked his client, Kristin M. Perry, “to be a lesbian?”

Mr. Olson’s question came during one of several attention-grabbing moments on the first day of the trial here on the constitutionality of Proposition 8, California’s voter-approved ban on same-sex marriage. It is a case that is expected to feature three weeks of evidence, experts and  if Monday was an indication  emotionally charged testimony on everything from the nature of gay sexuality to the nature of a romantic same-sex marriage proposal.

Ms. Perry, a 45-year-old child services professional who brought the case with her partner, Sandra B. Stier, 47, and a male couple, recounted how she had slowly fallen in love with Ms. Stier.

“I remember thinking that she was the sparkliest person I’d ever met,” Ms. Perry said, drawing giggles from the packed courtroom. When she told Ms. Stier of her feelings, she said, “she told me she loved me, too.” Ms. Perry proposed marriage in 2003, although same-sex marriage was illegal then in California and every other state.