"A Valediction: Forbidding Mourning" is a love poem, but Donne needs a whipping boy to compare to his all-powerful love. He uses lust. He kicks lust all over England in this poem as a way of showing just how much better his love is. Poor lust. It never stood a chance.
Lust is the opposite of love, if you think about it. Lust wants to take; love wants to give. Lust is skin deep, but love is body, mind, and soul. Donne calls our attention to the simple truth: when two people who are "in love" can't stand to be apart even for a little while, doesn't that tell us all we need to know about what their love is based on?
Donne is overstating his case here and it's a little ridiculous. I mean, are we supposed to discredit anyone's relationship simply because they cry when they have to be apart?
Donne overstates his case for good reason—he believes no one could have a love like his. Compared to his love, everything else is just shallow lust. So there.