Brian is intrigued by Leigh's request. He's not into commitment, even with a woman as attractive as Leigh, but he's also not the type to make a deposit and then disappear. If he goes along with her scheme, he's got one demand of his own: marriage before conception. He agrees to keep things clinical—he can get sex elsewhere—but having a wife at home will keep the husband-hunters at bay.
It seems like the ideal compromise—until they start falling in love.