Moving of th'earth brings harms and fears,
Men reckon what it did, and meant (9-10)
Lust is like an earthquake (or "moving of the earth"). It's a destructive force that attracts lots of attention and gets everyone riled up. Everyone is talking about the couple that's all over each other. That doesn't make it love.
Dull, sublunary lovers' love
(Whose soul is sense) cannot admit
Absence, because it doth remove
Those things which elemented it. (13-16)
This is the whole argument. Earthly lovers (who are really just lusting) only care about the senses—mainly touch. So when they have to part, they lose the only thing holding them together.
Inter-assurèd of the mind,
Care less eyes, lips and hands to miss (19-20)
Who needs lust when you have peace of mind and commitment in love? Not this couple, that's for sure.
Though I must go, endure not yet
A breach, but an expansion (22-23)
Donne calls lust dull, but here he also calls it crumbly. Love is gold—it can expand and stretch. But lust is like a brittle metal that gets pulled and quickly cracks and breaks. It wasn't one pure unit—it was two things stuck to each other, sucking face.