San Francisco ? Along with the other gays and lesbians in California, the more than 52,000 registered domestic partners still can’t get married. But starting today, if they split up, they’ll have to divorce.

If they have children, they will automatically receive a wide array of parental rights. Community property laws suddenly apply, just as for married spouses.

The changes are part of a law, put on the books by the Legislature and former Gov. Gray Davis in September 2003 but not going into effect until now, that greatly expands California’s 5-year-old experiment with domestic partnerships.

The law’s supporters and opponents agree it makes domestic partnership in California equivalent to marriage in almost all but name.

That puts the state once again at the forefront of the legal battles over gay unions. Same-sex couples affected by the law vastly outnumber those covered by Vermont’s system of civil unions or those who have wed in Massachusetts, the only state that allows gay marriages.

California’s registered domestic partners now share hundreds of state rights and responsibilities formerly granted only to married spouses.

In order to split, partners must legally divorce. Alimony could be ordered by a court. All assets acquired during the partnership are carved down the middle. Debts are shared, too. Both partners automatically receive the legal protections of parents along with the right to custody and the duty to pay child support.

“California is way ahead of the curve,” said Aimee Gelnaw, executive director for the Washington-based Family Pride Coalition, which promotes the interests of gay families. “The result of this in terms of national impact is that people will see that no one is hurt — that it’s in the best interest of all of us that parents and children and families are protected.”

Conservative organizations agree with her about the law’s sweep but not about its desirability. Robert Tyler, an attorney for the Alliance Defense Fund, a Christian legal group, calls it a “counterfeit marriage statute.” His group sued to block the law. It has lost so far but has an appeal pending.