Tommy Lee and his smoke show gf are coming clean -- they claim -- about their entry into the "Mile High Club" ... and, for once, the tale doesn't involve Tommy's famous junk.

When we got Tommy and Brittany Furlan Monday at the Commons at Calabasas, we got right to the bottom of the story we broke. They spent 20 minutes in the first class lavatory aboard an American Airlines flight last month.

Tommy's swordsman reputation and his penis being what they are, other passengers assumed ... y'know. Who wouldn't?

Well, Tommy and Britt tell us what was really going down behind closed doors ain't nearly as sexy as you'd hoped. It's their story, they're sticking to it.

But you don't have to buy it.

STUCK IN THE HEAD
TMZ.com