Jeanine and Nichole Soterwood have made the same emotional and financial commitments as most married couples. They share a name. They have stood before a justice of the peace and exchanged vows. They both own their house and are the parents of a 4-year-old son, Isaac.

But one thing about the Soterwoods is clearly not the same. They aren't married, because they can't be. Instead, they are among nearly 109,000 same-sex couples who live together in California - double the number of any other state. The most the Soterwoods could do is register with the state as domestic partners - a step roughly 43,000 California couples have taken, and that the Soterwoods moved specifically from Arizona to do.

As the gay marriage issue returns to center stage in California, same-sex registered partners like the Soterwoods have a direct personal stake in the issue. A look at the growing body of data on these couples shows a group that is larger in California than in any other state, and - perhaps not surprisingly - that looks more similar to married heterosexual couples in terms of affluence, parenthood and other social measures.

Like many same-sex couples, the Soterwoods don't want the similarities to end there.

"People are like, what does that mean, 'commitment ceremony'? Just like, what does that really mean, 'domestic partnership'?" said Nichole Soterwood, who like Jeanine is an engineer who works for Raytheon, the defense contractor. "Well, Advertisement people know what marriage means."

Activists say a majority of California's registered partners, and a significant portion of same-sex couples who live together but who aren't registered, would likely marry if the California Supreme Court reversed the ban on same-sex marriage.

"For many couples I know who are registered domestic partners, they see it as an intermediate step to winning full marriage equality," said Kate Kendell, executive director of the National Center for Lesbian Rights, a counsel for the plaintiffs in the same-sex marriage suit the Supreme Court will hear Tuesday.

That would create a very loud chorus of "I dos," although it might also create a backlash from those who believe gay marriage would undermine the institution. Roughly one in six unmarried partner households in the state are same-sex couples, among the highest share in the nation, according to 2006 U.S. Census Bureau data. About six in 10 of California's same-sex couples are gay men, while four in 10 are lesbians.

The Census Bureau's estimates of the number of same-sex couples is growing across the country - it's up 18 percent in California since 2000. Still, the number remains small compared with California's 6 million married couples, and experts say the higher estimates don't necessarily mean there are more same-sex couples.

"It's very likely that most of this is simply more accurate reporting," said Gary Gates, a demographer at the UCLA law school's Williams Institute. "It's certainly possible that with increased social acceptance, more lesbians and gay men are actually coupling and cohabiting, but given the magnitude of the increases, it's very unlikely that this is the primary cause."

A recent state survey of California's gay, lesbian, bisexual and transgender population primarily done to assess tobacco use also found that gay and lesbian couples who live together - particularly those who are registered partners - resemble married heterosexual couples in terms of affluence, race and other social measures.

The data undermines the stereotype that gay men eschew long-term relationships. Lesbians in California are more likely to partner - 64 percent share a home with a partner compared with 46 percent of gay men. But gay male registered partners in California have been together longer - an average of 12 1/4 years, compared with 8.9 years for lesbian registered partners, according to Gates' analysis of the state-funded tobacco survey.

Registered lesbian partners are much more likely to have children, however, with 34 percent of California female registered partners reporting children compared with just 2 percent of men.

California has about 14 percent of the nation's same-sex couples, slightly higher than the state's share of the total U.S. population, but census estimates of same-sex couples are growing more quickly in the Midwest and other places away from the coasts not traditionally seen as centers of gay life. The census does not measure the full gay and lesbian population, only the number of same-sex couples who live together.

California may have a higher proportion of same-sex couples than many states because of the early rise of large and influential gay communities in San Francisco and West Hollywood, communities that attracted those fleeing the hostility of the rest of the country, Gates said.

Devin Baker and Art Adams of San Jose, who along with the Soterwoods are among the roughly 1,300 registered partners in Santa Clara County, were in the clerk's office in San Francisco City Hall, their marriage application half-filled-out, when the state Supreme Court halted the same-sex marriages Mayor Gavin Newsom had begun in 2004.

Amid a tangle of photographers and family members, their new wedding rings in hand, Adams remembers a city official ordering, "You have to stop!"

Adams and Baker, both 43, watched as the heterosexual couple in line behind them got married on the spot they were to take their vows. Four years later, it remains a bitter memory for the two men, the only South Bay plaintiffs in the marriage lawsuit.

"It's just a very basic equality that values my relationship the same as it would any heterosexual relationship," said Baker, explaining his desire to marry. "You don't have to explain it; you don't have to excuse it; you don't have to give it different words. It's just the same."

Contact Mike Swift at mswift@mercurynews.com or (408) 271-3648.