"Wuv… twoo wuv." We know, we know—every discussion about marriage has to reference that scene from The Princess Bride. This book's not about marriage in the traditional sense, though—or even in the movie sense. Nope, it's after a much more metaphorical view of what marriage can be.
In this case, Blake is arguing—through his speaker—for a much more complex understanding of human nature. The best way to understand what he's after is to ask yourself two questions:
Question 1—Are people bad all the time, always? Question 2—Are people good all the time, always?
Of course, these two questions share just one answer: N-O. Of course people are more complex than "sinner" or "saint." So why do we have a religion that sends them off either to Heaven or to Hell, once they're time on Earth is spent?
That's the question that Blake's asking, anyway. And that's why we get the title The Marriage of Heaven and Hell. It's about bringing those seemingly contradictory elements of human nature together, because—guess what—they're both already a part of every human being.
Blake is not just trying to complicate our view of humanity, though. He's out to celebrate it. His idea is that, from this push and pull of good and bad impulses, we get vital energy. As he puts it: "Without contraries is no progression" (1.24). We actually need both elements to make us human, to make us creative and capable beings. And that's why, in this book's view, both Heaven and Hell need to rush on up to the altar and get hitched.