John Donne takes us all the way back to the Old Testament book of Genesis, which says that when two people are married two individuals become one new thing. Loyalty makes a lot more sense when disloyalty means betraying a part of yourself.
Though I must go, endure not yet
A breach, but an expansion, (22-23)
At first, this sounds like cheesy, groundless pillow talk. But, if you think about it, the speaker is pointing out how his impending absence will actually increase their love and loyalty. As they say, "absence makes the heart grow fonder…"
Yet when the other far doth roam,
It leans and hearkens after it, (30-31)
Loyalty is about give and take. No, Donne isn't being disloyal, but he will be straining their relationship by leaving. He praises his wife for being the one willing to stand firm at home and lean after him to keep them strong.
Thy firmness makes my circle just
And makes me end where I begun. (35-36)
Loyalty creates loyalty. Because Donne's wife is firm, it allows Donne to do what he needs to do. It also helps guide him back to her.