Honest Abe's sex life is the subject of the upcoming doc "Lover of Men: The Untold History of Abraham Lincoln" ... namely, his alleged romantic relationships with dudes.

Lincoln's sexual preferences are pieced together via interviews with dozens of scholars, plus never-before-seen letters and photos.

As one expert on the 16th president claims, "Lincoln probably slept in the same bed with more men than he did with women."

Of course, this isn't the first time there's been speculation Honest Abe was gay or bi. The topic's been explored for decades in magazine cover stories, books and other published works.

Abe's marital struggles with Mary Todd are well documented, and for 4 years he allegedly shared a bed with his male friend Joshua Fry Speed.

This film's synopsis says historical evidence of Lincoln's romantic relationships widens the "lens into the history of human sexual fluidity and focuses on the profound differences between sexual mores of the nineteenth century and those we hold today."