The Internet seems to think Lil Baby got caught in a sex act that would suggest he's into men -- but the dude's speaking up and calling BS ... while telling everyone to cool their jets.

The rapper is trending right now because of a viral video that's making the rounds -- which is sexually explicit and graphic ... depicting a man giving another man fellatio. It's one of those weird, fuzzy recordings of a recording type of clip ... the source of which is unknown.