"It wasn't like it was a big secret. It was just a cause I cared about and privately supported but not one that I had ever had an opportunity to speak out about in a way that would be useful," she tells Zap2it's Dish Rag.
Paquin, who's engaged to her True Blood costar Stephen Moyer, says she felt compelled to make the announcement, even though she's "not someone who endlessly talks about her personal life."
"Obviously, as someone who identifies as bisexual, those are issues I really care about – and frankly, I don’t see why everyone doesn’t care about them," she says.
And there was no hesitation on her part to come out. "When I was asked to participate in that PSA, it was just an obvious, 'Well, of course I will.' "