Not to get caught up in semantics ...perhaps I will first state the obvious, as the obvious is not exempt from the criticism of those who will debate that which is already understood by both, just for the sake of debate.

So, no ....marriage is not simply two people loving each other. 

A mom can love her child, a child can love their grandparent, and 'best friends' can love each other ...and in each case their is no claim of marriage, nor plans for it.  So, rather the reverse can be said also ...that two people loving each other does not in itself provide all we expect a marriage to be.

First of all, I believe that when people do get married, it's debatable whether it can be said that they love one another. 

I'd say they like each other a whole bunch ...but, they should be getting married because they are committing not to always like one another, but rather to commit to one another for their lifetime.

Yet, if one commits to love another for a lifetime, does that mean that he/or she will always be understanding and loving??

No, it does not!!

It means that each professes to not give up on the other.

If you felt you loved a person enough at one point in your life to marry them ...then it is not the marriage that is wrong, but the commitment to it. (And yes, I understand the exceptions ...with the list being rather long.  And I agree that there are certainly good reasons ...such as extensive abuse and violence.  Yet, there are bad reasons also.  That being said, this is not an intended discussion of past events ...we do move along in our lives, hopefully learning something along the way.  And we don't beat ourselves up over what we've learned.)