Stanza 5 Summary

Get out the microscope, because we’re going through this poem line-by-line.

They sat them down upon the yellow sand,
Between the sun and moon upon the shore;

These stoned sailors just plop down on the sand of the beach. As the speaker oh-so-poetically puts it, they sit down "[b]etween the sun and moon." That's a reference to the sunset we heard so much about in previous stanzas and the "full-faced […] moon" from line 7.
Our speaker is weaving together the drama of the sailors and the Lotos with the natural landscape he laid out so carefully for us.

And sweet it was to dream of Fatherland,
Of child, and wife, and slave; but evermore

While these sailors are sitting on the beach, they have happy dreams of their home ("Fatherland") and the children and wives and slaves they left behind. Hmm… one of those things isn't quite like the others is it? If you were stranded on an island, would you miss your slaves? We thought not. The thing is, these guys are ancient Greeks, so slaves were part of their households.
That's more of an explanation than an excuse, of course—slavery still doesn't seem real cool to us, even if you are an ancient Greek.
Be sure to notice the word "dream" in line 39. Images of sleeping and dreaming are absolutely everywhere in this poem.

Most weary seem'd the sea, weary the oar,
Weary the wandering fields of barren foam.

Even though they like thinking about home, everything about getting back there just seems exhausting to these Lotos-eating sailors. The idea of the sea makes them tired. Ditto rowing ("the oar") and the open ocean (poetically described as "fields of barren foam").
We can kind of see where these guys are coming from. If we were on a tropical beach with magical fruit to eat, we might not be that psyched about getting up and doing a bunch of rowing either.

Then some one said, "We will return no more";

This line marks a big shift in the poem. It's the moment where someone finally says what the Lotos-eating sailors have been thinking: now that they've tasted the Lotos, they don't feel like going home. They're putting up decorations and settling in for the long haul. This is the idea that drives the rest of the poem, and it's at the heart of the myth of the Lotos-eaters.

And all at once they sang, "Our island home
Is far beyond the wave; we will no longer roam."

Now the sailors start singing together, joining their voices to let everyone know they plan to stay put, to stop the roaming that brought them here in the first place. They are never going to see the "island home" they came from again. It's just too darn far away. Basically, a few bites of Lotos have turned these guys into an epic bunch of couch potatoes—albeit singing couch potatoes.