Madonna's Ex-Boyfriend Gag Alert -- KISS OF DEATH

We now know why Madonna broke up with her 26-year-old ex-boyfriend Jesus Luz ... the guy kisses ... like a St. Bernard.

The photo was taken today in Italy. We don't know who the unlucky girl is, but we're really hoping she brought a towel ... and some industrial strength disinfectant.

FYI, Madge and Jesus first got together in 2008 when Jesus was just 21 (and Madonna was 50) and split in 2010. They probably had some other problems too.