Robin Roberts -- No Secret She Was Gay

Robin Roberts -- who came out yesterday -- has been open about her sexuality for years ... she just didn't feel the need to go public until now.

Sources close to Robin tell TMZ ... everyone in her life -- both personally and professionally -- knew she was gay and in a 10-year relationship with Amber Laign. The 2 of them met in San Francisco through mutual friends.

Amber is a massage therapist who specializes in helping people with injuries.

As for their relationship ... our sources say Amber was utterly devoted to Robin when she went through both of her illnesses -- breast cancer in 2007 and Myelodysplastic Syndrome in 2012. Robin was extremely ill and we're told Amber was her rock.

As for why Robin came out yesterday ... we're told she never felt the need to tell the world, but at the end of such a difficult year she wanted to acknowledge what Amber did for her.