Moore, 49, said she had wanted to work with director Lisa Cholodenko since her 1998 feature "High Art," which also involves a lesbian relationship.
Cholodenko took nearly five years to complete the script, although with gay marriage such a hot topic in the United States, her timing turned out to be good.
"I don't see myself as an overtly political person," the director told reporters in Berlin after a press screening.
"What I do feel good about is that there's so much activity in the States right now about gay marriage. I think the timing of this film is quite interesting (although) it's not calculated.
"I feel like, as an artist, I've contributed politically."
Jules (Moore) and Nic (Bening) are trying to be good parents. The wealthy couple are proud of their daughter Joni's academic achievements and struggle to keep an open mind when they think their son Laser may be gay.
When they inadvertently discover Laser has contacted his biological father, the women feel threatened, but gradually warm to the handsome, laid-back epitome of California cool variously referred to as "the donor" and "the spermster."
Things turn sour, however, when Jules starts to fall for Paul (Mark Ruffalo) and control freak Nic begins to suspect.
The characters both despise and embody social mores of modern California, be it health drinks, organic food, composting or a liberated attitude to sex.