Bill mused, with the hundreds of books that have been written about Trump, no one has cracked the mystery ... who is he banging? As he says, Trump's been a dog all of his adult life, talking about how powerful people could simply grab a woman by the p***y and have his way. Bill's convinced Trump hasn't changed his ways, yet no one has addressed it.

Bill quickly rules out Melania, so then who?

Now here's the deal. The "Real Time" host is really not that interested in D.T.'s love life, and here's how we know. Toward the end, Maher suggests another theory ... Trump has the desire but not the ability, because he's impotent.

Now on the surface this may sound like high school humor, but that misses the point ... Bill knows Trump is as braggadocious as they come -- no pun intended -- and he will find it irresistible to defend his "manhood."

When you think about it, exactly how does Trump answer? Bill has put Trump in a box.  Again ...