Heidi Klum -- Yes, I'm Banging the Bodyguard

Heidi Klum just admitted on national television ... she's getting it on with her bodyguard -- but the supermodel insisted she never cheated on Seal.

Heidi just opened up on Katie Couric's talk show, claiming she "never looked at another man" while she was with Seal -- despite Seal's accusations she boned the family bodyguard behind his back while they were married.

Heidi said she's known the bodyguard for 4 years and, over that time, they've become very close. Heidi said they are now "seeing" each other.