"Bara" is short for "Barazoku", an influential Japanese gay magazine which shut down publication in late 2004. 2chan came up with the abbreviated name for their "gay art" imageboard (which has since been discontinued).
Western, gay male anime and manga fans then appropriated the word to contrast with yaoi. Both are homoerotic anime-style art; the difference is while yaoi is gay-male art produced by women for the consumption of women, bara is created by men, for men. Consequently, bara-style art tends toward more exaggerated, overtly masculine physiques and stereotypically masculine behavior.
Bara in this context has nothing to do with "rose", which is what the word means in Japanese.
Break me off a piece of that hawt Asuma bara action.
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