Ben Affleck flew home from Detroit to celebrate his 42nd birthday with the wifey and kids ... and low and behold, he found Jennifer Garner was pregnant! Just kidding ... not really ...

Could it be that even though Ben's out of town working on Batman, he somehow knocked up Jen? Or is it that she's not pregnant? Or is it that someone else ...

Kidding again ...

The happy couple celebrated Ben's big day with daughters Violet and Seraphina and son Samuel and stepped out in the morning to grab a coffee (probably decaf for Jen) and then Ben gassed up the ol' Chevy.

There's just something about a hot guy, and a hot car ... we just MELT. Can you image how many girls must throw themselves at Ben? It must be SO difficult to stay faithful!