The Forgetting Sarah Marshall star accepted Shepard's marriage proposal at the end of 2009, but the couple has yet to make it official - and Bell, who counts gay actor Zachary Quinto as one of her best friends, reveals there's a reason why its nuptials will be some time off.
She tells The Advocate magazine, "The reason we're not rushing to get married is because I don't feel appropriate taking advantage of a right that's denied to my best friends.
"That's why we've been so hesitant. Dax and I have talked about it a lot, and this issue is very important to both of us. We're just standing up for what we believe in. Period."
Bell's Heroes co-star Quinto 'came out' as a gay man last year (11) and the actress admits she is "wildly proud" of her pal for choosing to live life in the open.
Gushing about Quinto, she adds, "He has known exactly who he is since the day I met him."
Washington, D.C. will become only the seventh U.S. state to legalise same-sex unions when the recently-passed bill comes into effect in June (12).
Bell and Shepard aren't the only celebrities to show their support for gay marriage - Brad Pitt previously announced he and his longtime partner Angelina Jolie would not become man and wife until everyone in America had the right to wed. He has since admitted he's feeling the pressure from their six children to tie the knot.