Brian McKnight & Sons -- Let's All Talk About Dirty Sex

Brian McKnight's sons have no problem talking about hardcore sex in front of their dad -- in fact, they RAP about it ... in a brand new music video about porn.

TMZ broke the story ... Brian hosted 15 porn stars and one little person at his L.A. mansion last month, where YouPorn producers shot the music video for McKnight's extremely explicit tribute song to the XXX website.

Now, the music video has finally surfaced -- and 22-year-old Brian McKnight Jr. and his 19-year-old brother Niko both star in it, rapping verses about having sex with women.

Here's a quick example -- "F***ing you on the carpet / Giving you rug burns, now it's your turn." And that's pretty tame compared to the rest.

Meanwhile, their dad's lyrics address everything from amateur porn to anal sex.