Over a casual dinner, Rachel Kaplan turned to her boyfriend and asked, "You're open for a prenup, right?" Darren Waldohlz, 38, a partner in a successful speed-dating business, admits he was caught off guard. But he said he realized a prenuptial agreement would protect the house he owns, too. "It's not that I plan to get divorced," says Kaplan, 23, a single mom from Fort Lauderdale, who has a sizable inheritance. "I have to protect myself and my daughter." While men are still more likely to seek a prenup, "women are becoming a more dominant force," said attorney Alan Braverman,...