What is love?
Love is the free self-giving of the heart. To have a heart full of love means to be so pleased with something that one emerges from oneself and devotes oneself to it. A musician can devote himself to a masterpiece. A kindergarten teacher can be there wholeheartedly for her charges. In every friendship there is love. The most beautiful form of love on earth, however, is the love between man and woman, in which two people give themselves to each other forever. All human love is an image of divine love, in which all love is at home. Love is the inmost being of the Triune God. In God there is continual exchange and perpetual self-giving. Through the overflowing of divine love, we participate in the eternal love of God. The more a person loves, the more he resembles God. Love should influence the whole life of a person, but it is realized with particular depth and symbolism when man and woman love one another in marriage and become "one flesh" (Gen 2:24).
How is sexuality related to love?
Sexuality must not be separated from love; they must go together. The sexual encounter requires the framework of a true, dependable love. When sexuality is separated from love and is sought only for the sake of satisfaction, one destroys the meaning of the sexual union of man and woman. Sexual union is the most beautiful bodily, sensual expression of love. People who look for sex without love are lying, because the closeness of their bodies does not correspond to the closeness of their hearts. Someone who does not take his own body language at its word does lasting damage to body and soul. Sex then becomes inhuman; it is degraded to a means of obtaining pleasure and degenerates into a commodity. Only committed, enduring love in marriage creates a space for sexuality that is experienced in a human way and brings lasting happiness.
(From the Catholic Youth Catechism)