Seal -- I Wish Heidi Wouldn't 'Fornicate with the Help'

Seal opened up in a shocking way last night, accusing Heidi Klum of banging her bodyguard during their marriage and alleging that's why they broke up.

The singer says ... by "fornicating with the help," Heidi clearly shows she lacks class.

Klum has been seen with the bodyguard more frequently lately ... the two were spotted walking alone together in New York City just a few weeks back (see below).

Heidi separated from Seal in January and filed for divorce in April. As we first reported, Heidi's story is that she couldn't handle Seal's explosive temper. At the time they separated, we're told Seal was desperately trying to save the marriage.

Things change.