Marriage is the union of two people. The lines agree, but what's with that word "surely"? It sounds like the speaker is trying to convince herself of her union with her husband, or rather it almost seems like she doesn't totally believe they are one. Do we detect a hint of doubt?
If ever man were loved by wife, then thee; If ever wife was happy in a man (2-3)
Notice that the first two lines say essentially the same thing: wife loves man, wife is happy with her man. The sequence – "if ever man… if ever wife" – makes us think that we're going to hear about both partners, but our sly speaker is really just talking about herself. These lines aren't actually about her "dear and loving husband" at all.
My love is such that Rivers cannot quench, Nor ought but love from thee, give recompense (7-8)
These lines are very symmetrical. The first talks about "my love," the second "love from thee." They're like mirror images of each other, which points to the harmony that is the essence of the speaker's marriage. At the same time, the lines rhyme imperfectly, so perhaps not everything about their marriage is as flawless as we're led to believe.
Then while we live, in love lets so persevere, That when we live no more, we may live ever (11-12)
The end of the poem echoes its beginning by using the word "ever" one last time. Do you think that this balance is meant to imitate the harmony that is the cornerstone of a successful marriage? Or is something else afoot?