The phrase "closet homosexual" refers to someone who is gay but keeps their sexual orientation a secret and does not openly share it with others. Full definition
Rich closeted gay men: Often looking for both male and female Sugar Babies, rich closeted homosexual men and bisexual / curious men prefer discreet arrangements with gay partners. (blog.seekingarrangement.com)
He is giving closeted homosexuals an example of somebody who is not afraid anymore. (si.com)
As the third season begins, we see that Weiner is committed strongly to going in this same direction with closeted homosexual Salvatore Romano. (metacritic.com)