In Philadelphia, why do you think Andrew Beckett decided to conceal his sexual orientation and his AIDS diagnosis from his peers at work?
During the trial, we learn the reason why Andrew Beckett felt the need to conceal his sexual orientation and his contraction of AIDS from his colleagues. The homophobic jokes and targeting of gay people that Andrew heard "behind close doors" leads Andrew to believe that he could not be forthcoming without suffering professional rejection. While this comes out in the trial, it is something that the viewer understands over the course of the narrative. The law firm is not very inclusive, reflecting the epitome of exclusion and wealth. For Beckett, it becomes clear that he is a cog in this configuration, one who wishes to advance professionally. This is not going to be accomplished in a setting that is exclusive and protective against the interests of inclusion.
The film shows the world of the corporate law firm as one in which tolerance is absent. Beckett realizes this and because of it, he is not inclined to share his sexual identity and diagnosis of AIDS with people at work. He understands this and recognizes that this is the reason why he is dismissed from his position.