Donne wrote some pretty saucy poems, but for all his playboy bravado, he stayed married to the same woman until her death. As a religious man, he cherished faithfulness to one's spouse. "A Valediction: Forbidding Mourning" is a love poem, but it's also a poem of praise to his wife's faithfulness. It is her faithful, enduring love that holds their relationship together, seeing as how, you know, he's not letting the shirt touch his back in his mad rush out the door.
Donne emphasizes his wife's loyalty, but he also is subtly requiring her to do nothing but stay at home and pine for him until he comes back. Good times for her.
The poem takes an extremely high view of marriage. Their love is committed for better or for worse. (We're guessing that it's about to get worse, for the wife anyway.)