Is it true that iambic pentameter is "natural" to English? If so, why?

I expect the strained rhymes in Dante are merely from the fact that it's translated. If you're writing original poetry, you have much more freedom to change things around to make them rhyme than you do if you're translating poetry, and care about keeping it moderately faithful to the original.

Anyway, Dante's terza rima scheme is pretty much iambic pentameter with an unusual (for English) rhyme scheme¹. Shelley wrote some great poetry in this scheme which rhymed perfectly well (e.g. Ode to the West Wind).

¹ Each line ends with a feminine (two-syllable) rhyme, rather than a masculine (one-syllable) one, but that's just because of the way Italian words are accented.

Great question! Yes, many claim that the best prose that scans as iambic. I question the "pentameter" part, though: to me, most of the best English prose is in tetrameter.

I'm sadly disappointed that this site
Should give its answers in prosodic ways
When of the poet's art we choose to write,
And rhyme and rhythm's secrets must display.

Does English ever to the iamb tend?
Not all perhaps, but much of what is there
To five full feet of short-long beats does lend
A certain credence to that theory bare.

But "natural beat" doesn't nat'rally meet the requirements of which the words speak.
Its rhythm's in three not in two, don't you see? So perhaps it's "dactylic" we seek.
And this verse, though amateur, is no pentameter, four feet then three is its bent.
So ask not "Who knows?" Maybe anything goes. But that's more than enough now, I'm spent.

The most natural English prose meter is the limerick kind that you hear here. It's really a cinch to produce in a pinch and it always reads simpler than Shakespeare. This meter is based on a system. It's three tones that come in a rhythm. Two low tones surround a high tone that's found at some regular places within them.

.-..-..-.
.-..-..-.
.-..-
.-..-
..-..-..-.

Dr. Seuss when he wrote, and he wrote for a while, used a rhythm that's much like the limerick in style. It's not quite a limerick, though they're both the same kind. It's a high tone that comes with two low tones behind.

..-..-..-..-
..-..-..-..-
..-..-..-..-
..-..-..-..-

Why are these systems so easy? Because low tones in English are common. You can write blank verse too that scans much alike, but lacks rhymes to help readers to see this.

English is naturally spoken Iambicly, that is we tend to pair stressed and unstressed syllables. Such a pairing is an Iamb, from the Greek for foot, Iambic meaning step. Like left-right,left-right.

But we do not tend to speak in pentameter.

Pentameter is a specific measure of syllables, that is five sets of paired syllables, which tend to be pairs of one stressed and one unstressed syllables. So Iambic pentameter merely means 10 syllables which tend to alternate in the way that they are stressed.

Actually, this is an interesting question, and I think I may have an answer. I am writing a book called Shakespeare's Numbers that explores the nature of Shakespeare's "iambic pentameter" and you are not amiss in questioning the "natural iambicity" of English. English is no more iambic than it is trochaic. The idea that its rhythms are iambic probably arise from the fact that the language is largely comprised of monosyllabic words. When speaking strings of monosyllabic words the normal alternation between short and long syllables accentuates the iambic rhythm, making English seem iambic. And when arranged by an unskillful poet in unending strings of iambs, yes, the language seems iambic. However, natural English can't be said to be iambic if half the time (or more) it's not iambic. My examination of Shakespeare's verse yields the observation that his blank verse was actually constructed to flatten lyricism in service of creating a mirror-like imitation of natural speech for the drama. Shakespeare's verse is as uniambic as it is iambic: it contains just as many "iambic inversions" as it does iambs, and speaking onstage in "iambic pentameter" mangles his verse as it was intended to be spoken, since the actor is attempting to instill lyricism, as he speaks, into verse from which it has been removed. As far as translating rhyme schemes between languages, it should be frowned upon; the manner in which a language can be lyrically rhymed with itself usually doesn't translate into a language of a different structure. If you're interested in these issues the best place to start is the long ignored The Arte of English Poesie published in 1589.

I did some research on this topic, finding answers ranging from that English doesn't fit this rhyme, to natural English following this meter, to sources saying “subject-object-verb” arrangement makes this sort of rhyme scheme work.

The most convincing piece of evidence claimed that rhyming was the norm until the early 1800’s (see: Romantic Era), but quickly fell from favor since the rhythm can feel “forced”. But then, there are examples from some famous playwrights (David Mamet, Aaron Sorkin) arguing that this way of writing is most natural. They use it in their dialogue, and everyone agrees how musical their lines are.

My conclusion is that it is natural, but a lost (or dying) art. For example, writing this entire answer using (almost) perfect iambs wasn’t hard. And I would argue that this answer, although quirky, doesn’t have the feel of forcing rhyming. But maybe that's just me.