The multi-hyphenate host and entertainer was darting through LAX Monday, when a photog tracked him down and asked him the million-dollar question that's been on everyone's mind since last week ... namely, whatcha think of Bre confessing to doing the deed with MBJ???

Remember, this is something BT proudly spilled on the Season 7 premiere of "Selling Sunset" ... when she and the other ladies were sitting around fantasizing about Hollywood's hunks.

Somebody threw out Mike's name pretty quickly, but Bre was immediately unimpressed ... 'cause she revealed that she'd already been there and "done that" with the guy.

Her pals were all shocked ... but Bre kept it even more real, saying she's slept with all her favorites -- which obviously ended up including Nick too. Speaking of him ... he's on camera here reacting to the news, and whodathunkit -- the dude's not really sweating this!

Check out his response yourself ... as he wisely points out, we all have a past -- and whatever Bre did with Mike was waaaay before him, so he doesn't give it a 2nd thought.

Of course, he and Bre share a child together -- so you'd figure he might be a little bent out of shape on this ... but he's really not. Happy-go-lucky Nick ... what else is new???