In western country ,marriage is not necessary to share love with two partners,But in arab and many religions need marriage to be necessary to validate the relation and love and sex sharing,so the marriage and love still be discussed,
The second argument you often hear is that marriage is a declaration of love. It's about "showing" we're settled, our partners are "off the market", and we're in a position to build a family. Most of this, however, is a display for others. Plenty of monogamous couples maintain stable, healthy relationships without rings or certificates to "prove" loyalty.
Indeed, who are we trying to prove our love to? Our proof should be our treatment of each other: anything else is addition, not basis. There is more to be worried about if we need to "secure" someone, like a raging animal, with a ring or certificate or other public stamp.
Furthermore, as high divorce rates show, being tied to one person doesn't work out for many, especially for the rest of our lives. Compromises can be made. Couples now swing, maintain open marriages, and so on. But this should only make us question why we're still devoted to the "one true love" ideal in the first place.