There are so many dimensions to this oneness, certainly there is the aspect of sexuality, but so much more! Think in terms of the Creator combining two parts---like a lock and key,or a violin and bow, in marriage, the relationship will not work unless the two parts are working in harmony. The two parts work to complement each other.
One way that Christianity frames the idea of marriage is that it is for life. Take note that there is a difference between how Churches view divorce: some don't admit it at all; others allow it in the case of adultery, while others will grant divorce for less serious issues, but we can agree that all Christian Churches allow divorce rather reluctantly.
Christianity views divorce more like a surgical operation, a separating or dissecting of the body, Jesus has this to say about the dissolution of a marriage: "what therefore God has joined together, let not man separate (Mark 10:9).
Love for each other is certainly one of the reasons that two people get married, but it is not to be the only reason for remaining married. People "fall out of love," but if love was the only reason for staying married it would leave no room for "the contract" or promises made before others and God.
Do these words sound familiar?
In the presence of God, our family and friends, I offer you my solemn vow to be your faithful partner in sickness and in health, in good times and in bad, and in joy as well as in sorrow. I promise to love you unconditionally, to support you in your goals, to honor and respect you, to laugh with you and cry with you, and to cherish you for as long as we both shall live.
To those who have taken the marriage vow, recommit your energy to those vows; both before your spouse and before your God.