People who claim to not be homophobic yet marginalize gay people or constantly enforce the notion that gay people are minority as opposed to treating them equally, in everyday talk.
Putting "the" before "gays" (saying "the gays") is marginalizing.
Saying "that's gay" or "******" is marginalizing.
People that come out as trivial **** as a joke is marginalizing.
Telling a gay person how to live their life is marginalizing.
etc.