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Ronda Rousey tells TMZ Sports ... she believes transgender MMA fighter Fallon Fox has an "unfair advantage" over naturally born women -- but she'd still kick her ass anyway.

Rousey was at LAX when we asked about Fallon -- a post-op transgender who was born a man and now fights with the women.

FYI -- Fox has won her last two fights ... and beat her last opponent so badly, she reportedly suffered a concussion, a broken orbital bone and required 7 staples in her head (video below).

Rousey -- the reigning female UFC champ -- says she's done a lot of research on the situation ... and ultimately feels Fox needs to be fighting with the men -- especially with the damage she's inflicting.