Nathan Fillion has dated women in the past and has never openly come out as being gay. He has brought dates to movie premieres and award shows, and there has been much speculation as to whether or not he is married.
Fillion talked about painting his girlfriend's toe nails on the April 16, 2014 "Conan" show. He also talked about her getting her master's in marine biology, but he seems to keep a low profile. Most sites post his sexuality, when applicable, as straight. There are only a few interviews in which Fillion mentions dating anyone, and they are past relationships with women.