(CBS NEWS) -- The nation's most influential pediatrician's group has endorsed same-sex marriage, saying a stable relationship between parents regardless of sexual orientation contributes to a child's health and well-being.

The new policy of the American Academy of Pediatrics, published online Thursday, cites research showing that the parents' sexual orientation has no effect on a child's development. Kids fare just as well in same-sex or straight families when they are nurturing and financially and emotionally stable, the academy says.

Separately, a new national survey shows the nation's views on same-sex marriage are more favorable -- in large part because of a shift in attitudes among those who know someone who is gay or became more accepting as they got older of gays and lesbians.

The Pew Research Center poll also finds that a large group of younger adults who tend to be more open to gay rights is driving the numbers upward.

The issue has grabbed the national spotlight recently with the public embrace of same-sex marriage by Democrat Hillary Rodham Clinton and Republican Sen. Rob Portman of Ohio.

The pediatricians' academy says a two-parent marriage is best equipped to provide that kind of environment. The new policy says that if a child has two same-sex parents who choose to marry, "It is in the best interests of their children that legal and social institutions allow and support them to do so."

The policy cites reports indicating that almost 2 million U.S. children are being raised by same-sex parents, many of them in states that don't allow gays to marry.

Officials with the academy said they wanted to make its views known before two same-sex marriage cases are considered by the U.S. Supreme Court next week.

"We wanted that policy statement available for the justices to review," said Dr. Thomas McInerney, the academy's president and a pediatrician in Rochester, N.Y.

The pediatricians' stance is not surprising. They previously joined other national groups, including the American Medical Association, in supporting one of the Supreme Court cases that contends the Defense of Marriage Act is unconstitutional. The academy also previously supported adoption by same-sex parents.

The academy's statement notes that several other national health groups have supported same-sex marriage. Those are the American Academy of Family Physicians, the American Psychiatric Association, the American Psychological Association and the American College of Nursing.

Dr. Ben Siegel, a Boston pediatrician and chairman of an academy committee that developed the new policy, said its focus is on "nurturing children. We want what's best for children."

Commenting on the new national survey, the Pew center's director, Michael Dimock, says, "We've certainly seen the trend (in support of same-sex marriage) over the last ten years. But we're now really in a position to talk about the combination of generational change and personal change that have sort of brought the country to where it is today."

Overall, the poll finds 49 percent of Americans favor allowing gay men and lesbians to marry legally, and 44 percent opposed to the idea. That's more people now favoring same-sex marriage than opposing it. A decade ago, 58 percent opposed it and a third supported it.

The 49 percent who now support same-sex marriage includes 14 percent who say they have changed their minds.

When asked why, almost one-third say it's because they know someone who is gay -- a family member, friend or acquaintance. A quarter said their personal views have changed as they thought more about the issue or just because they've grown older and more accepting.

One of those polled said, "My best friend from high school is a gay man, and he deserves the same rights," adding that his friend and a partner "are in a committed relationship."

Another person attributed the shift in attitude to "old fashioned ignorance," and said, "I grew up a little bit."

Just 2 percent overall said their views have shifted against same-sex marriage.

Another major factor in the long-term shift in the public's view: the so-called millennial generation of young adults born since 1980 -- today's 18- to 32-year-olds who entered adulthood in the new millennium. The survey finds 70 percent of millennials favor same-sex marriage.

Same-sex marriage has long been an issue of partisan political debate, but it resurfaced recently with Clinton and Portman declaring their support, and with the Supreme Court preparing to take up the issue.

On Monday, Clinton announced her support for same-sex marriage, lining up with other potential Democratic presidential candidates who favor it.

In an online video released by the same-sex rights advocacy group Human Rights Campaign, Clinton says gays and lesbians are "full and equal citizens and deserve the rights of citizenship."

And last week, Portman reversed course and said he now supports same-sex marriage. He said he had a change of heart after he learned one of his sons is gay. "I have come to believe that if two people are prepared to make a lifetime commitment to love and care for each other in good times and in bad, the government shouldn't deny them the opportunity to get married," he wrote in an op-ed in The Columbus Dispatch. His reversal makes him the only Republican in the Senate to back same-sex marriage.

The Supreme Court will hear arguments next week in a challenge to a provision of the Defense of Marriage Act, the federal law that defines marriage as between one man and one woman. It's also reviewing California's Proposition 8 ban on same-sex marriage.

The Pew Research findings are based on a survey of 1,501 adults nationwide conducted Mar. 13-Mar. 17. The poll has a margin of error of plus or minus 3 percentage points.