This psychological-theological graduate thesis speaks of infatuation (crush), love, sexual intercourse, and purity of the body during love relationships between a Christian young man and girl. As a Catholic, every young man is called to preserve the dignity of his body because the body is the temple of the Holy Spirit which is in everyone (1 Corinthians 6:19), until he decides through the period of love relationship that he is willing to crown his love with marriage and only in marriage become one body (Rev. 2. 24) with his partner. Topics of this thesis are familiarizing between a young man and a girl and the decision to start their own love relationship, differences between infatuation and love that young people often do not distinguish because in love heart prevails over mind that is silent and in love heart and reason cooperate together. This paper will also look at how and in what way to behave sexually in a relationship both with oneself and with the partner, and what does sexuality has to do with Christ. After the relationship is mature, and the couple crosses over from the steps of infatuation to the steps of real love, the next thing to do is to see if they will respond to each other in the future, whether they are mature, capable and ready for marriage. Ultimately, this thesis will try to associate psychology of love with theology of body, summarize the psychological preparation for marriage of a young man and girl, as well as give answers to questions such as: what is the difference between infatuation(crush) and love? How to recognize true and sincere love? What is the role of purity in love relationship and why to live the purity of the body before marriage? What are the consequences of having a sexual relationship before marriage? How to choose an ideal marriage partner? When to decide for marriage and how to prepare for it?