Heterosexual unmarried couples in CA can't register as domestic partners (except for couples over the age of 62). That's why Google's new policy applies only to same-sex couples who are registered.
The registry itself is an attempt, by the state, to extend many of the rights and benefits available to married heterosexual couples to same-sex couples whose marriages are not recognized by the state. But there are numerous rights and benefits it does not (and can not, because of federal law) extend. A heterosexual married Googler can opt to extend Google's employee health insurance benefit to that Googler's spouse, and a homosexual domestically partnered Googler can opt to extend Google's employee health insurance benefit to that Googler's domestic partner. However, the cost of doing so for the gay Googler will be significantly higher, because the benefit the gay Googler gets (for the domestic partner) is considered taxable income. The benefit the heterosexual married Googler gets (for the spouse) is not taxable income.
All Google is doing is making up the cost differential. It isn't discriminating against anybody. You could certainly make the argument that the state is discriminating in at least two ways: 1) not recognizing the marriages of same-sex couples; 2) not allowing unmarried heterosexual couples under the age of 62 to register as domestic partners in lieu of getting married. But I doubt the state would even bother with a domestic partnership registry if the marriages of same-sex couples were on the same footing with the marriages of opposite-sex couples. Really, what would be the point?
Gay people have already outed themselves by registering their domestic partnership. Further, they out themselves by paying for the health insurance benefit for their domestic partner. Google's new policy isn't requiring any additional "outing."
I agree that it would be much simpler if the state would simply recognize same-sex marriages, rather than create a whole parallel system for dealing with same-sex couples. (Note: Gay people can, and do, get married all the time, in both religious and civil ceremonies, and have been doing so in this country for 40-50 years or more. The state can not and never has prevented gays from marrying; it does, however, refuse to accord those marriages the same status it accords opposite-sex marriages.) Another option would be to ignore the term "marriage" altogether and simply treat anyone who wants to register their relationship with the state as domestic partners or civil partners. But these things are out of Google's control, and I think Google is doing the right thing here in redressing one imbalance that is within its power to redress.