Theresa Volpe and Mercedes Santos' 8-year-old daughter, Ava, has learned about the civil rights movement of the 1950s and 1960s in school. So her school friends understand when she tells them her two moms are fighting for their civil rights to marry -- it's just hard to explain it to grown-ups sometimes, Volpe and Santos say. "This is really for our kids," said Volpe, who along with Santos are plaintiffs in a lawsuit demanding same sex marriage in Illinois. "It's important for them to be treated fairly. We're just as much a family as any other family." Illinois could become the next U.S. state to legalize gay marriage with a bill set to be introduced in the state Senate this week. Gay marriage supporters in Illinois say they plan to press for approval in a Democratic-majority legislature in the next few days. President Barack Obama endorsed the proposal to legalize gay marriage in his home state. If it passes, Illinois would be the 10th state to approve marriage between same-sex couples. And Volpe and Santos could get what their siblings have already had -- a real wedding.