This is a good, general question that requires you home in on a series of adjectives to explain the marriage. After examining these passages below, see what you can identify as characteristics of Brutus' and Portia's union.
The first interaction we see between husband and wife is complaint. Portia comes to him and says,
Y'have ungently, Brutus,
Stole from my bed; and yesternight at supper
You suddenly arose and walk'd about,(250)
Musing and sighing, with your arms across;
And when I ask'd you what the matter was,
You stared upon me with ungentle looks.
She accuses him of being "ungentle" -- inconsiderate and rough with her -- and for ignoring her query as to "what the matter was," and then giving her a rude look.
What does it say about a marriage where one person can openly confront another and say, "Hey, you've done me wrong when I was wondering if you were okay?" You might settle on such descriptive adjectives as "open," "forthright," "confrontive," and other such terms.
Portia ends her speech with, "Dear my lord,
Make me acquainted with your cause of grief."
Note the use of "dear" and "make me acquainted." She is loving and concerned, so you might argue that she is both affectionate and devoted, or compassionate and concerned. (You see that you have a number of options in your descriptions, and after examining more text, you can be more precise.)
Let's look at Brutus' response to her. First he tries to put her off -- "I am not well in health, and that is all" (remember, he is a Stoic, so he is the least likely to moan and complain; never mind the fact he is embroiled in a conspiracy. But back to the marriage.) First he says this, but eventually, does he cave? Yes. By the end of the conversation, he has capitulated:
...by and by thy bosom shall partake
The secrets of my heart.
All my engagements I will construe to thee,(320)
All the charactery of my sad brows.
Now we see that Brutus is honest as Portia is. So this marriage is characterized by honesty.
Look at Portia's plea prior to that, which is more evidence of devotion at work and concern. She even kneels. What is Brutus' response to her, and how does his response show his feelings for her? Would you say that these two are equals, in a society where men tended to be more dominant (both ancient Rome and Renaissance England)?