Michael Lohan Marries Baby Mama -- Lindsay Left in the Dark

Michael Lohan has finally made an honest woman out of GF Kate Major ... but no one alerted his kids.

Lindsay Lohan's old man got hitched Oct. 30th in a private ceremony at Hillsboro Beach in Florida. It was so private neither Lindsay nor Mike Jr. were invited ... or even told.

Here's the thing. When Lindsay finally did find out ... she didn't care.

We're told Michael's motivation was his soon-to-be-born child. This is #2 for Michael and Kate.

They've had a rough ride with multiple mutual arrests. Hopefully "till death do us part" comes naturally.