The number of unmarried heterosexual couples living together reached a record 6.4 million in 2007, or 10% of all hetero couples who share a home, according to the Census Bureau. It's a snapshot of the changing American family. Some 2.5 million of the couples—45.5%—are raising at least one biological child of either partner, and 1.3 million—21%—have a child together, reports USA Today.
Living together in the US doesn't often indicate a long-term commitment, but it has become "part of the life course," said a sociologist. "It's something people do that leads to somewhere. If it doesn't lead to marriage, it leads to splitsville."