For years, Julie Brinton's days have been filled with sippy cups and skinned knees, and the endless push of getting three young children through dinner time, bath time, bedtime. By the time she crawls into bed each night, she has one thing on her mind: zoning out to an hour or so of TV. But some nights, her husband, Rob, reaches over to rub her shoulders and offer her a back rub. And then Ms. Brinton thinks: "Has it really been three weeks? I guess we should probably have sex." "I will do it for him," says Ms. Brinton, 34,...