How can the person being controlled and the person in control be of equal rank?!

    Nice question indeed!  To me, such an occurrence is essential to any relationship between Snape and another person.  Dominated and Dominator have to be carefully balance AND are interchangeable! I know some of you will tell me it's impossible, but I think so! Please read what follows:

      From my own observations, an equilibrated couple is always made of one dominator and one dominated. These are called patterns in psychology. Examples are : manipulator/manipulated,  oppressor/victim, etc. This is what I think we all refer too when we talk about dominator/dominated: the duality and the complementarity. HOWEVER, a dominator may turn out to be the dominated in some fields or circumstances! They work like the ying/yang of Chinese philosophy. Here are my explanations:

    Let's take our favourite subject as an example: Snape!  I do believe Snape would be the dominator.  Why? Being a Slytherin is a good giveaway. But there's more!  As Isabelle mentioned before, Snape must be quite the apprehensive type when it comes to trusting other people. He's been used by Voldemort (that much is obvious) and surely by some other people (Death Eaters, family, Slytherins or even the Marauders).  In the novels, you see how Snape hates to be humiliated, not believed by Dumbledore, not trusted, etc.  Does that look good in a dominated's résumé? No. On the other hand, it doesn't mean Snape, like Isabelle argued, wants to be an all powerful dominator (that would rather fall into the sado-maso category). Hence, so he would get rid of the too submissive woman in question very rapidly or so he would not even pay any attention to her!  Don't forget that Snape has been humiliated and shunned away by the wizarding world. Moody's reaction to him shows that much.  Therefore, a real submissive woman would also tend to submit to mainstream opinions as well. And what's the opinion of every wizard towards Snape? "Don't touch, might poison you...or worst!" Therefore, neither Snape nor the submissive woman would truly like each other.  And Snape has proved times and again how he hates or despises gullible folks, incompetent, scared Nevilles, etc. He's impatient with them. So, Snape doesn't want or need to be a powerful dominator.

    Ok, so Snape is the dominant type and he wouldn't like a submissive woman, but where can the balance be? And why are roles interchangeable? Like I said, a couple needs a dominator and a dominated. However, you have to imagine this concept as a two-sided scale (like a balance). The dominant one doesn't need to take all the weight to himself!  That's the worst that can happen to someone. Imagine a manipulator who manipulates everything in his partner: that's the recipe for chaos (trust me on that one, the partner looses even her personality and all freedom)  Like I said, everything has to be in balance according to the standards of the person. Snape likes perfection and rules. Therefore, he needs someone that can stand up to these standards. Surely, he'll be the dominator in these fields. On the other hand, his lady will have some standards herself!  Standards may be too harsh a word. Let's say strong and weak points in the person.  It's very human to want someone to help you with your weak points and not denigrate you for them!  When you think of Snape's weak points, you automatically think of his lack of sociability, his stubbornness, his anger and his brooding attitude. Thereof, his lady would have to be the dominant person in these points.  If two insecure people are together, it's no good is it?  They'll just scare one another, unless they both recognize their weaknesses and work on it!  But that's pretty rare. Good couples are those who can balance each other out. They have common traits and interests, they have differences that balance each other out.  That view is as old as time really! So if you're still following me here, you'll realize that Snape needs someone who stands up to his standards AND that balances his weak and strong points at the same time. And after all, why can't Snape feel like being dominated sometimes or in some fields?  It doesn't diminishes what or who he is at all!  He needs challenge, someone who will not be stopped by mere appearances. Someone who believes him... Just someone to give up some of its power and put it in his hands because she trusts him.