Is it normal to think that it is not necessary to marry the one you love?

Actually, It is "Romantic Love" that dies on its own in a marriage. When the sexual desires wane, the excitement turns to "everyday" life and they say "you're not who I married" is when the illusion is finally gone.
This can be a time when love can grow in a caring/understanding/accepting way. This is what TRUE LOVE is.

And I wonder why the divorce rate is high.

I don't think it's necessary to do anything do codify love, but it's not sensible to say that marriage kills love. I'm sure married people who fall out of love with each other would have fallen out of love whether they were married or not, and there are plenty of happy, long married people.

Married couples are supposed to "love" each other, but they don't have to be "in love". All that is necessary is that the two take care of each other for as long as they live.

Well ok yes you do not have to marry someone you love but I think people would prefer being tied a sexual partner they can love, tolerate and actually want to have sex with. However if you are marrying for another reason that is entirely up to you.

I agree you can be in love and in a committed relationship without getting married. I'm not so sure about marriage killing love...