Your partner is female, and I am gathering that you are female, as well as the co-worker. Sorry, if I am wrong, but the sexes of all parties involved is somewhat ambiguous.

Are you sure that the co-worker is out? Are you sure that the co-worker is gay? (This is where knowing the sexes and orientations of everyone involved would help, because yes, in this case, it could make a difference in the advice given.)