In the poem "To My Dear and Loving Husband," what is the rhetorical situation?

A rhetorical situation is defined as the intersection between three elements: an exigence (issue, challenge, or conflict), an audience (who can be persuaded and inspired to bring about change), and a set of constraints (people, institutions, personal beliefs, traditions that can help influence the audience).
So, we're going to discuss this in the context of Anne Bradstreet's poem "To My Dear and Loving Husband."
Exigence: Anne wants to persuade her husband to stay the course of their happy union. By all indications, Anne was a happily married Puritan woman. In her time, Puritans thought of marriage as a practical necessity. People didn't marry solely for love. If mutual love did develop after marriage, it was considered a perk.
The reality of the situation was this: Children were needed to replenish the labor supply, which was necessary to sustain the colonies. For its part, sex was an important procreative tool.
In Anne's case, marriage was not just a practical necessity: she actually loved her husband. The first few lines of the poem highlight this fact.

If ever two were one, then surely we.
If ever man were loved by wife, then thee.
If ever wife was happy in a man,
Compare with me, ye women, if you can.

Because Anne loved her husband, she would allow nothing to estrange her from her husband. In this poem, she is trying to persuade her husband to stay the course of the union, no matter what happens.
Audience: Anne's husband. This appears to be a very personal poem.
Set of constraints: In the poem, the constraints are Anne's Puritan beliefs, marital conventions, and the shared love between the couple.
Puritans believed that marriage was a character-building institution and that, if nurtured properly, it conferred spiritual and material benefits to the husband and wife. Divorce was frowned upon and could only be obtained for reasons such as adultery or desertion.
So, Anne aims to persuade her husband to retain love in their marriage, to "persevere," so that their love can be eternal. Anne suggests that, by staying the course, her husband (and also herself) will reap "manifold" rewards from God.
https://www.thoughtco.com/rhetorical-situation-1692061