Ronda Rousey -- Fallon Fox Has Unfair Advantage ... But I'd Still Beat Her Ass!

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.