It has more to do with spiritual equivalence than if they are a Christian or not, that part's obvious. People are at different points in their walk with Christ though. You might have two electricians, one a journeyman, and the other just starting out, and they're not going to work that well together because the newbie is going to be asking questions or the Journeyman has to correct them on something. It makes the newbie feel anxious about their work, and the Journeyman annoyed. It's similar with Christian relationships. in a relationship, the woman is to be the man's most trusted advisor. It's hard to be an advisor if the person you're advising doesn't know what you're talking about, or the advisor has nothing to say. It's really hard for a man to choose a direction to go If he has no idea whom or what to consult, or when he can't rely on his other half to do anything other than look pretty. A yoke is used as it sybolizes a binding between the two that is very strong. Where one person goes, the other will be forced to go, and they can either be in equilibrium, or face abrasion and wear. If one person walks slower than the other, the cart that's being pulled, we'll call it "life", will just keep going in circles. People who are brand new to the faith will be like trying to maintain the same pace as a child. People who take bits and pieces, or aren't following in the faith will be like walking with the crippled. Granted, a non-believer will be like a dead weight at the back of the cart. A person just being a Christian is the most important foundation, but it goes beyond that if you want a smooth (er) running machine. This goes for non-romantic relationships as well.