Further Fudanshi terminology research:

according to Japanese wikipedia (machine translated)

Fudanshi , all- name fudanshi (Japanese: fudanshi / Fudanshi Fudanshi ) , yasaku fudanshi ( fudanshi / fukei Fukē ), fujoshi ’s lyricism, Sokushi-kiri BL , Bara , Yaoi ’s works men [1] . “Rotten brother” is a model of “parents” [1] , appeared in 2002, first seen on PINK channel ( Japanese : PINK channel ) 801 board [2]

translated from:

腐男,全稱腐男子(日语:腐男子ふだんし Fudanshi),也作腐兄(腐兄ふけい Fukē),為基於腐女衍生而來的詞彙,即指喜歡BLBaraYaoi作品的男性[1]。「腐兄」一詞則是模仿「父兄」[1],2002年出現,最初見於PINK channel(日语:PINKちゃんねる)的801板[2]

So from this, we can glean that the term “fudanshi” first gained usage around 2002, on Japanese imageboards. Specifically PINK channel which was an adults only imageboard.